Allow exceptions to bubble up to CMS

CiviCRM provides CRM_Core_TemporaryErrorScope::useException() which allows us to disable errors from within CiviCRM and throw exceptions instead. However, these exceptions are caught by CRM_Core_Invoke::invoke() and passed into CRM_Core_Error::handleUnhandledException($e) where PHP is subsequently killed.

There's a good use case for the CMS to be handling the exception, ie. by providing a nice pretty error page, logging to CMS logger, etc.

This issue tracks issues with allowing exceptions to bubble up into the CMS.