Force a hard rollback whenever we trip on a fatal error ..

we can potentially create a custom group and have an error in the table creation. this causes a fatal error, but does not rollback the transaction (since by default all transactions commit)

we should do a rollback for all transactions whenever there is a hard failure. check:

http://forum.civicrm.org/index.php/topic,2682.msg11581.html#msg11581

for more details