adding stubs for 500 or 503 pages

Not entirely sure how to wire these up to the app.
I noticed for 404 errors, we have a couple hooks in fxa-content-server.js:

- [server/bin/fxa-content-server.js:42](https://github.com/mozilla/fxa-content-server/blob/master/server/bin/fxa-content-server.js#L42)
- [server/bin/fxa-content-server.js:88](https://github.com/mozilla/fxa-content-server/blob/master/server/bin/fxa-content-server.js#L88)