Improve cleanup/error handling

[schmiddy]

A few gripes with our current error handling:

• pgut_atexit_push() is called relatively late (after step 1 is complete), which is probably OK with current git master, but is a bit dodgy for the concurrent-ddl-guard changes
• Instead of calling ROLLBACK unconditionally, we should use pgut_rollback() which checks PQtransactionStatus(conn)
• Instead of using printf() plus exit() for reporting fatal error messages, an elog or ereport ... ERROR seems more appropriate
• Errors encountered in reorg_one_table(), e.g. for conflicted triggers, shouldn't be treated as fatal -- pg_reorg might be operating on a whole database, and should be able to continue processing other tables