Live merge failed during Merge Status command, Destroy Image command or Destroy Image Check command.
If this is an active image live merge, the top volume is left as illegal at the end of the failed live merge.
The reason of the failure in those commands is, usually, network timeout when executing Vdsm verbs. In that case, we keep trying executing the verbs until succeeding.
Live merge process will not fail if there is an issue calling Vdsm and the top volume will not be illegal.