It was brought up in #562 that in case we receive an OAuth error that we
don't know about, `specific_oauth_error` will fall through with a `nil`,
then picked up by `specific_api_error` which will always try to handle
the error as if it were a `Hash` (even if we know it's not!) and thus
lead to typing problems at runtime.
This patch throws a generic `OAuthError` in cases where a code comes
back that we don't recognize. I'm still crazy about the fact that we
don't have a better way of recognizing an OAuth error in particular, but
it should do the trick.
24a1704f05