Capture exceptions swallowed by rails

[robertclancy]

ActionDispatch's ShowExceptions swallows exceptions and stores the cause
in a rack env variable "action_dispatch.exception". Use this to report
on any exceptions swallowed by ShowExceptions.

[nateberkopec]

Can you just change this line to say 'should capture rails errors when ActionDispatch::ShowExceptions is enabled'?

[seanlinsley]

This should be a setting. For example, we set rescue_from ActiveRecord::RecordNotFound globally in our app. We don't want those showing up in Sentry.

[robertclancy]

Any exceptions rescued in a controller using rescue_from do not end up in the "action_dispatch.exception" env variable.

As far as I can tell, the only place that env variable is set is in the ShowExceptions middleware, which catches exceptions not rescued in a controller.

[nateberkopec]

rescue_from doesn't use ShowExceptions. This LGTM.

[greysteil]

This is causing some issues for us - our app specifies an exceptions_app which handles things like parameter parsing errors. We don't want those rescued exceptions to be reported to sentry - our app has already done the right thing and returned a 400 (for example).

My hunch is that the best way forward on this one is to make this a setting. Alternatively we could delete the action_dispatch.exception env variable once the exception has been handled by our exceptions app, but that feels unintuitive. @nateberkopec / @seanlinsley - any thoughts?