what are responses when things *don't* check out?

[Pomax]

Looking at https://github.com/mozilla/id.webmaker.org/blob/develop/docs/oauth.md#get-user-data, what is the response when a token doesn't check out? I assume an error object is sent back, but what does it look like?

And there's probably a few more routes where the error response is missing.