dmllib: more consistent err handling for execute_sql() and others

dmllib: more consistent err handling for execute_sql() and others

Silent errors in execute_sql() have been causing us lots of grief.
Turns out that if you have dblogerror set, it _will_ write it to the
logs.

DMLlib wasn't consistent. Most "modern" codepaths were calling both
debugging() and the dblogerror idiom. So we make all calls to
$db->Execute() consistent.

(Some exceptions remain, mostly for hardcoded SQL, such as the SET
statements when we connect...)