dbcheck: Abandon dbcheck if we get an error during a transaction

dbcheck: Abandon dbcheck if we get an error during a transaction

Otherwise, anything that the transaction has already done to the DB will be left in the DB
even despite the failure.  For example, if a fix wrote to the DB, but then failed a post-write
check, then the fix will not be unrolled.

This is because we do not have nested transactions in TDB.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12178

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Mon Aug 29 12:46:21 CEST 2016 on sn-devel-144