SQL error MUST NOT happen... so stop the execution of if an SQL error is

SQL error MUST NOT happen... so stop the execution of if an SQL error is
found.

Why ?
I've found too many invisible SQL errors during the last few weeks
(invisible == not shown in a backtrace because of a page redirection). SQL
errors are evil (hum), they should NEVER happen: a query that is allowed to
fail can always been rewritten in a way errors are ignored (e.g. INSERT
IGNORE).

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>