Action Dispatch ShowExceptions

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 a parameter on initialization of ShowExceptions. Every time there is an exception, ShowExceptions will store the exception in env["action_dispatch.exception"], rewrite the PATH_INFO to the exception status code, and call the Rack app.

In Rails applications, the exceptions app can be configured with config.exceptions_app, which defaults to ActionDispatch::PublicExceptions.

If the application returns a response with the X-Cascade header set to "pass", this middleware will send an empty response as a result with the correct status code. If any exception happens inside the exceptions app, this middleware catches the exceptions and returns a failsafe response.