Disable broken mako debugging

As pylons provided mako debugging has been broken for some time and
makes things even worse, this commit disables it entirely.

The broken custom handle_mako_error resulted in error tracebacks
stopping at the place where mako error handling would normally take
place, instead of getting down to the actual error. This is especially
relevant if the error didn't occur in the template, but in other code,
which was called from within a template.