Avoid shadowing ignored errors listAllBuckets() (#5524)

It can happen such that one of the disks that was down would
return 'errDiskNotFound' but the err is preserved due to
loop shadowing which leads to issues when healing the bucket.

cmd/xl-v1-healing.go

@@ -300,7 +300,7 @@ func listAllBuckets(storageDisks []StorageAPI) (buckets map[string]VolInfo,
 			if errors.IsErrIgnored(err, bucketMetadataOpIgnoredErrs...) {
 				continue
 			}
-			break
+			return nil, nil, err
 		}
 		for _, volInfo := range volsInfo {
 			// StorageAPI can send volume names which are
@@ -316,7 +316,7 @@ func listAllBuckets(storageDisks []StorageAPI) (buckets map[string]VolInfo,
 			buckets[volInfo.Name] = volInfo
 		}
 	}
-	return buckets, bucketsOcc, err
+	return buckets, bucketsOcc, nil
 }
 
 // ListBucketsHeal - Find all buckets that need to be healed