debug help for ticket 1533

have tried to find a way to minimize the impact on correctly configured sites, but there is nothing here that would raise an exception with L0 set and no rows found, it would just not execute the loop and then behave strangely in a completely different place where the original cause can no longer be inferred - so either we force it into the exception (like i do it here), or we accept tickets like 1533 and tell people to RTFM ;)

can no longer see the point with recovering from this either - a debug message has much less visibility than a crash and could even be turned off, so better to put the message right into the traceback and refuse to work until fixed?