Add MWExceptionRenderer class and decouple DBError

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