rebase: warn if state directory cannot be removed

rebase: warn if state directory cannot be removed

If rebase --quit cannot remove the state directory then it dies. However
when rebase finishes normally or the user runs rebase --abort any errors
that occur when removing the state directory are ignored. That is fixed
by this commit.

All of the callers of finish_rebase() except the code
that handles --abort are careful to make sure they get a postive return
value, do the same for --abort.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>