fix multiple bugs

weed/storage/store_ec.go

@@ -45,6 +45,8 @@ func (s *Store) MountEcShards(collection string, vid needle.VolumeId, shardId er
 				EcIndexBits: uint32(shardBits.AddShardId(shardId)),
 			}
 			return nil
+		} else {
+			return fmt.Errorf("%s load ec shard %d.%d: %v", location.Directory, vid, shardId, err)
 		}
 	}