Add better logging / error capturing of Canvas Studio admin auth errors

[robertknight]

Improve the debug-ability of Canvas Studio admin auth errors.

• For errors reported in response to an external request, raise the error as an ExternalRequestError (instead of HTTPBadRequest or SerializableError) so that it gets logged in Sentry

• For other errors related to admin auth, log details so that we can see these events in Papertrail

For context see this Slack thread.

[robertknight]

Closing as this is not a priority for the moment. I will re-open if needed.