This middleware rescues any exception returned by the application and calls an exceptions app that will wrap it in a format for the end user.
The exceptions app should be passed as parameter on initialization of
ShowExceptions. Every time there is an exception,
ShowExceptions will store the exception in env, rewrite the PATH_INFO to the exception status code and call the Rack app.
If the application returns a “X-Cascade” pass response, this middleware will send an empty response as result with the correct status code. If any exception happens inside the exceptions app, this middleware catches the exceptions and returns a failsafe response.