Log a helpful message if exchange_code_for_access_token() times out and the fake UAA is enabled

[toolness]

It's a pretty minor use case, but I tried enabling the fake UAA provider on a cloud.gov deployment just now and the callback endpoint seemed to "hang", eventually resulting in a timeout message from cloudfoundry.

[toolness]

Oh... um... it is probably because the endpoint was unable to call back to its own server instance because WEB_CONCURRENCY defaults to 1 and I didn't manually set it to anything else. Which we've already documented.

[toolness]

That said, I wonder if it'd make a sense to add a timeout to the endpoint and have it log a helpful error message if the fake UAA is enabled.