notmuch-restore: fix return value propagation

notmuch-restore: fix return value propagation

Previously notmuch_restore_command returned 0 if tag_message returned
a non-zero (failure) value. This is wrong, since non-zero status
indicates something mysterious went wrong with retrieving the message,
or applying it.

There was also a failure to check or propagate the return value from
tag_op_list_apply in tag_message.