Add MWExceptionRenderer class and decouple DBError

• This handles the work of showing exceptions so that MWException does not have too.
• Simplify the DBError classes to regular Exception classes. Lots of pointless prettification has been removed, but DBConnectionError still gets the usual special treatment of a fallback page and Google form.
• Remove hacky file cache fallback code that probably did not work.
• Make MWExceptionHandler::report() wrap MWExceptionExposer::output().
• Make MWException::runHooks() wrap MWExceptionExposer::runHooks().

Change-Id: I5dfdc84e94ddac65417226cf7c84513ebb9f9faa