message: Unregister sync_session_states::Error session when it's closed
diff: @@ -541,6 +541,7 @@ void SyncSession::handle_error(SyncError error)
  case ProtocolError::bad_client_file_ident:
  case ProtocolError::bad_server_version:
  case ProtocolError::diverging_histories:
  + next_state = NextStateAfterError::inactive;
  update_error_and_mark_file_for_deletion(error, ShouldBackup::yes);
  break;
  case ProtocolError::bad_changeset:
  