XL: Implement ignore errors. (#2136)

Each metadata ops have a list of errors which can be
ignored, this is essentially needed when

  - disks are not found
  - disks are found but cannot be accessed (permission denied)
  - disks are there but fresh disks were added

This is needed since we don't have healing code in place where
it would have healed the fresh disks added.

Fixes #2072

fs-v1-multipart.go

@@ -85,7 +85,7 @@ func (fs fsObjects) listMultipartUploads(bucket, prefix, keyMarker, uploadIDMark
 			// For any walk error return right away.
 			if walkResult.err != nil {
 				// File not found or Disk not found is a valid case.
-				if walkResult.err == errFileNotFound || walkResult.err == errDiskNotFound || walkResult.err == errFaultyDisk {
+				if isErrIgnored(walkResult.err, walkResultIgnoredErrs) {
 					eof = true
 					break
 				}