reveal GH authentication failure in `watch`

[emilymcafee]

If GithubAccessToken is set, but the token doesn't have enough permissions, the error from Github ends up hidden here, causing the next line to fail with an unhelpful error.

I wonder if we could inspect the response from the Github API to make sure it hasn't errored before proceeding.

[emilymcafee]

Probs could do this along w https://github.com/mapbox/ecs-conex/issues/8