Oracle can set error_reporting to 0

While testing MDL-42912, I discovered that error_reporting could be set to 0 in some circumstances, hiding all the possible reporting.

This is caused by Oracle's query_start() and query_end(). While the first save the current error_reporting, and the latter reset it, it can happen that query_start is called twice in a raw, leading to an error_reporting of 0 from that point.

Identified on 2.4.