add stack trace processor and refactor introspection processor to reduce code duplication

Good call. After some further digging, it seems what I was describing comes from a combination of Symfony's HttpKernel ExceptionListener and the MonologBundle having a handler configured to email critical errors. This procossesor was then defined as a tagged service on the critical handler. The refactoring of the IntrospectionProcessor also modifies the returned value of debug_traceback(), so I'll need to fix that if there's consensus I should continue working on this PR