Gracefully handle unknown formats

[zachmargolis]

Why: Bad input should not cause us to throw exceptions

[jmhooper]

Part of me wonders if we should render a 400 here instead? Maybe not b/c we don't have a template and all that setup for a 400?

[zachmargolis]

I have no strong opinions, we could even render the "not found" page with a 400 status code if you think that would be better?

[jmhooper]

Maybe? Now that I'm looking at the bug this fixes, I'm thinking a 404 makes a bit more sense.

[zachmargolis]

ok we'll stick with 404 for now. Thanks!

[monfresh]

In Rails 5, ActionController::UnknownFormat will return a 406 HTTP status code, which is the correct code for this scenario. By rescuing the error and returning a 404 instead, we are not accurately capturing errors.

If the goal of this PR was to prevent on-call people from being awakened in the middle of the night, I think the solution is to exclude 406 errors from New Relic.

Also, note that the New Relic errors linked to in the issue this PR was meant to fix are all ActionView::MissingTemplate errors, and the reason those errors are throwing a 500 is because production is still using Rails 4.2.8. In Rails 5, we won't see ActionView::MissingTemplate anymore, except in some cases where we call render in the controller, which is a separate issue I'm fixing.

[zachmargolis]

I think Rails 5 has already been in production: https://github.com/18F/identity-idp/releases/tag/2017-08-30T210357

[monfresh]

Perhaps the wrong tag was deployed? The stack trace doesn't lie. New Relic clearly shows Rails 4.2.8.

[brodygov]

Good catch finding the wrong tag thing!