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

xl-v1-common.go

@@ -67,7 +67,7 @@ func (xl xlObjects) isObject(bucket, prefix string) (ok bool) {
 			return true
 		}
 		// Ignore for file not found,  disk not found or faulty disk.
-		if err == errFileNotFound || err == errDiskNotFound || err == errFaultyDisk {
+		if isErrIgnored(err, walkResultIgnoredErrs) {
 			continue
 		}
 		errorIf(err, "Unable to stat a file %s/%s/%s", bucket, prefix, xlMetaJSONFile)