storage/xl: Return errVolumeAccessDenied if disks cannot be accessed. (#1621)

Fixes #1614

object-common-multipart.go

@@ -312,7 +312,7 @@ func listMetaBucketMultipartFiles(layer ObjectLayer, prefixPath string, markerPa
 				"marker":    markerPath,
 				"recursive": recursive,
 			}).Debugf("Walk resulted in an error %s", walkResult.err)
-			// 'File not found' or 'Disk not found' is a valid case.
+			// File not found or Disk not found is a valid case.
 			if walkResult.err == errFileNotFound || walkResult.err == errDiskNotFound {
 				return nil, true, nil
 			}