There are a few cases in which people can end up with confusing error

messages when pointing RBTools to the wrong Review Board URL. For

instance, if they've pointed to something that has a /api/ URL, but

doesn't have a Review Board root API payload, they can get an

AttributeError. If they've pointed to an expired RBCommons team,

they'll get "Unexpected API Error: 404".
This updates our handling to better check for 404s and that the root API

payload has what we'd expect. If checks fail, we check if it's an issue

with an RBCommons-specific URL, providing instructions to help, and then

fall back on an error message explaining what's wrong and what URL

they're using.

Tested variations on RBCommons URLs, both valid and invalid.
Tested a non-Review Board URL that had a JSON /api/ endpoint.
Tested URLs that didn't look at all like Review Board servers.