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

autoload.php

@@ -767,6 +767,7 @@
 	'MWDocGen' => __DIR__ . '/maintenance/mwdocgen.php',
 	'MWException' => __DIR__ . '/includes/exception/MWException.php',
 	'MWExceptionHandler' => __DIR__ . '/includes/exception/MWExceptionHandler.php',
+	'MWExceptionRenderer' => __DIR__ . '/includes/exception/MWExceptionRenderer.php',
 	'MWGrants' => __DIR__ . '/includes/utils/MWGrants.php',
 	'MWHttpRequest' => __DIR__ . '/includes/HttpFunctions.php',
 	'MWMemcached' => __DIR__ . '/includes/compat/MemcachedClientCompat.php',