Fix visibility of error handlers

I've hit this error when $wgDBuser didn't have access to $wgDBname.
I've also checked all other error handlers in core.

An alternative approach would be to use an anonymous function but it doesn't support $this until PHP 5.4 so it would be messier.

Change-Id: I89db2e8c88b84d9b53c4b828ac0208b05a3d3783
Merge "Fix visibility of error handlers"