Propagate return values from _MergeInto back from MergeInto

Merging a ticket into itself now displays an error ("Merge
failed. Couldn't set EffectiveId") and no logner leaves a backtrace in
the logs (due to calling both ->Rollback _and_ ->Commit)

Fixes I#26407.
Provide a better error message for the case of self-merging
Merge branch '4.2/self-merge-error' into 4.2-trunk