Fix regression in admin router when no route matches (#8409)

When `mc admin user add` is attempted in gateway mode without
etcd setup, NoSuchBucket error is returned instead of MethodNotAllowed.
Regression from commit - e48005ddc7

cmd/admin-router.go

@@ -144,5 +144,6 @@ func registerAdminRouter(router *mux.Router, enableConfigOps, enableIAMOps bool)
 	adminV1Router.Methods(http.MethodGet).Path("/kms/key/status").HandlerFunc(httpTraceAll(adminAPI.KMSKeyStatusHandler))
 
 	// If none of the routes match, return error.
+	adminV1Router.NotFoundHandler = http.HandlerFunc(httpTraceHdrs(notFoundHandler))
 	adminV1Router.MethodNotAllowedHandler = http.HandlerFunc(httpTraceAll(versionMismatchHandler))
 }