DeletedObjects: Return objects on lock failure (#10874)

Return objects when locking fails.

<details>
<summary>Panic</summary>

```
: 2020/11/10 04:15:55 http: panic serving 10.10.62.153:44858: runtime error: index out of range [0] with length 0
: goroutine 363537270 [running]:
: net/http.(*conn).serve.func1(0xc019232780)
:         net/http/server.go:1801 +0x147
: panic(0x1cadd60, 0xc001719260)
:         runtime/panic.go:975 +0x47a
: github.com/minio/minio/cmd.criticalErrorHandler.ServeHTTP.func1(0xc0121d1200, 0x210cda0, 0xc0141940e0)
:         github.com/minio/minio/cmd/generic-handlers.go:781 +0x1a8
: panic(0x1cadd60, 0xc001719260)
:         runtime/panic.go:969 +0x1b9
: github.com/minio/minio/cmd.objectAPIHandlers.DeleteMultipleObjectsHandler(0x1e71ce8, 0x1e71cc8, 0x2108420, 0xc0192328c0, 0xc0121d1400)
:         github.com/minio/minio/cmd/bucket-handlers.go:465 +0x2490
: net/http.HandlerFunc.ServeHTTP(...)
:         net/http/server.go:2042
: github.com/minio/minio/cmd.httpTraceAll.func1(0x2108420, 0xc0192328c0, 0xc0121d1400)
:         github.com/minio/minio/cmd/handler-utils.go:353 +0x158
: net/http.HandlerFunc.ServeHTTP(...)
:         net/http/server.go:2042
: github.com/minio/minio/cmd.collectAPIStats.func1(0x2108420, 0xc019232820, 0xc0121d1400)
:         github.com/minio/minio/cmd/handler-utils.go:380 +0xed
: net/http.HandlerFunc.ServeHTTP(...)
:         net/http/server.go:2042
: github.com/minio/minio/cmd.maxClients.func1(0x2108420, 0xc019232820, 0xc0121d1400)
:         github.com/minio/minio/cmd/handler-api.go:132 +0x33b
: net/http.HandlerFunc.ServeHTTP(0xc00271d590, 0x2108420, 0xc019232820, 0xc0121d1400)
:         net/http/server.go:2042 +0x44
: github.com/minio/minio/cmd.redirectHandler.ServeHTTP(0x20e2180, 0xc00271d590, 0x2108420, 0xc019232820, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:192 +0x156
: github.com/minio/minio/cmd.customHeaderHandler.ServeHTTP(0x20e1060, 0xc0141a22b0, 0x21083e0, 0xc01814d2e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:751 +0x162
: github.com/minio/minio/cmd.securityHeaderHandler.ServeHTTP(0x20e0fc0, 0xc0141a22c0, 0x21083e0, 0xc01814d2e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:766 +0x1d6
: github.com/minio/minio/cmd.bucketForwardingHandler.ServeHTTP(0xc0121c7a40, 0x20e1120, 0xc0141a22d0, 0x21083e0, 0xc01814d2e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:624 +0xbf
: github.com/minio/minio/cmd.requestValidityHandler.ServeHTTP(0x20e0f20, 0xc01814d280, 0x21083e0, 0xc01814d2e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:608 +0x42a
: github.com/minio/minio/cmd.httpStatsHandler.ServeHTTP(0x20e10c0, 0xc0141a2300, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:536 +0xe4
: github.com/minio/minio/cmd.requestSizeLimitHandler.ServeHTTP(0x20e0fe0, 0xc0141a2310, 0x50004000000, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:68 +0xd4
: github.com/minio/minio/cmd.requestHeaderSizeLimitHandler.ServeHTTP(0x20e10a0, 0xc01814d2a0, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:93 +0x1b7
: github.com/minio/minio/cmd.crossDomainPolicy.ServeHTTP(0x20e1080, 0xc0141a2320, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/crossdomain-xml-handler.go:51 +0x82
: github.com/minio/minio/cmd.browserRedirectHandler.ServeHTTP(0x20e0fa0, 0xc0141a2330, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:276 +0x68
: github.com/minio/minio/cmd.minioReservedBucketHandler.ServeHTTP(0x20e0f00, 0xc0141a2340, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:344 +0xb8
: github.com/minio/minio/cmd.cacheControlHandler.ServeHTTP(0x20e1020, 0xc0141a2350, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:303 +0x1ce
: github.com/minio/minio/cmd.timeValidityHandler.ServeHTTP(0x20e0f40, 0xc0141a2360, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:414 +0x3ca
: github.com/minio/minio/cmd.resourceHandler.ServeHTTP(0x20e1160, 0xc0141a2370, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:516 +0xab
: github.com/minio/minio/cmd.authHandler.ServeHTTP(0x20e1100, 0xc0141a2380, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/auth-handler.go:502 +0x2e7
: github.com/minio/minio/cmd.sseTLSHandler.ServeHTTP(0x20e0ee0, 0xc0141a2390, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:802 +0x79
: github.com/minio/minio/cmd.reservedMetadataHandler.ServeHTTP(0x20e1140, 0xc0141a23a0, 0x210cda0, 0xc0141940e0, 0xc0121d1400)
:         github.com/minio/minio/cmd/generic-handlers.go:139 +0x1b7
: github.com/gorilla/mux.(*Router).ServeHTTP(0xc00073fb00, 0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         github.com/gorilla/mux@v1.8.0/mux.go:210 +0xd3
: github.com/rs/cors.(*Cors).Handler.func1(0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         github.com/rs/cors@v1.7.0/cors.go:219 +0x1b9
: net/http.HandlerFunc.ServeHTTP(0xc0009aece0, 0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         net/http/server.go:2042 +0x44
: github.com/minio/minio/cmd.criticalErrorHandler.ServeHTTP(0x20e2180, 0xc0009aece0, 0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         github.com/minio/minio/cmd/generic-handlers.go:784 +0x85
: github.com/minio/minio/cmd/http.(*Server).Start.func1(0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         github.com/minio/minio/cmd/http/server.go:101 +0x258
: net/http.HandlerFunc.ServeHTTP(0xc000dc4080, 0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         net/http/server.go:2042 +0x44
: net/http.serverHandler.ServeHTTP(0xc000764c60, 0x210cda0, 0xc0141940e0, 0xc0121d1200)
:         net/http/server.go:2843 +0xa3
: net/http.(*conn).serve(0xc019232780, 0x2114720, 0xc03381f6c0)
:         net/http/server.go:1925 +0x8ad
: created by net/http.(*Server).Serve
:         net/http/server.go:2969 +0x36c
```
</details>

cmd/erasure-server-sets.go

@@ -574,7 +574,7 @@ func (z *erasureServerSets) DeleteObjects(ctx context.Context, bucket string, ob
 		for i := range derrs {
 			derrs[i] = err
 		}
-		return nil, derrs
+		return dobjects, derrs
 	}
 	defer multiDeleteLock.Unlock()