decom: Ignore not found buckets (minio#509) (minio#21023)

When decommissioning is started, the list of buckets to decommission is
calculated, however, a bucket can be removed before decommissioning reaches
it. This will cause an infinite loop of listing error complaining about
the non-existence of the bucket. This commit will ignore
errVolumeNotFound to skip the not found bucket.

Author: vadmeste

cmd/erasure-server-pool-decom.go

@@ -1025,7 +1025,7 @@ func (z *erasureServerPools) decommissionPool(ctx context.Context, idx int, pool
 						go decommissionEntry(entry)
 					},
 				)
-				if err == nil || errors.Is(err, context.Canceled) {
+				if err == nil || errors.Is(err, context.Canceled) || errors.Is(err, errVolumeNotFound) {
 					break
 				}
 				setN := humanize.Ordinal(setIdx + 1)