Add error message or warning in the Sourcegraph Webui when there is an error fetching Github api call

Created by: northyg

Feature request description

Error message or warning in the Sourcegraph Webui when there is an error fetching Github API. For example, if I forgot to allow the GitHub token to access private repositories, it should return an error indicated why.

Is your feature request related to a problem? If so, please describe.

The problem is related to install and configuration. It can be easy to miss a step such as the one described above, and not realize what was missing.