status-indicator: Display all syncing errors from repo-updater

Created by: mrnugget

This is a follow-up to #5319 and implements the improved error reporting as described in this comment: https://github.com/sourcegraph/sourcegraph/pull/5319#discussion_r316627988

The user visible changes:

• The types of StatusMessages have been changed so that there is now an ExternalServiceSyncError to which an external service is attached and the new SyncError without an external service
• The SyncError will now be produced whenever the syncing process in repo-updater fails, even if that doesn't involve an external service. That means we now, for example, display errors that originate in the database. That's something customer's ran into in the past but that was never visible.

Test plan: manual testing, go test, cd web && yarn test --runInBand StatusMessagesNavItem.test.tsx