Make external service syncing errors visible to site-admins

Created by: mrnugget

As part of improving the site-admin experience in 3.8 I want to revisit the topic of displaying external service errors in global status indicator.

A previous attempt was https://github.com/sourcegraph/sourcegraph/pull/4804 but it wasn't merged, because I ultimately decided that I didn't like the approach to error handling in repo-update.

I now want to revise that PR but

1. change the GraphQL definition of statusmessages
2. define errors in repo-updater in a way that allows us to differentiate between different classes of errors: network error, authentication error, repo not found, etc.
3. improve the way the error messages are displayed