XL: Handle object layer initialization properly.

Initialization when disk was down the network disk
reported an incorrect error rather than errDiskNotFound.

This resulted in incorrect error handling during
prepInitStorage() stage.

Fixes #2577

cmd/object-common.go

@@ -183,7 +183,7 @@ func xlHouseKeeping(storageDisks []StorageAPI) error {
 
 			// Cleanup all temp entries upon start.
 			err := cleanupDir(disk, minioMetaBucket, tmpMetaPrefix)
-			if err != nil {
+			if err != nil && err != errDiskNotFound {
 				errs[index] = err
 			}
 		}(index, disk)