fixed theorem lookup code in Isar proof reconstruction

tuning

refactor code

tuning

more robust future_proof result with specific error message (e.g. relevant for incomplete proof of non-registered theorem);

collective errors from use_thys and Session.finish/Goal.finish_futures -- avoid uninformative interrupts stemming from failure of goal forks that are not registered in the theory (e.g. unnamed theorems);

more uniform group for map_future, which is relevant for cancel in worker_task vs. future_job -- prefer peer group despite 81d03a29980c;