Improve Geo logging of repository errors

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2968

From the points taken from the issue above:

Consolidate all Geo-related activity into a common log (e.g. geo.log)

This was practically done, but I encountered a couple of places where we weren't doing this.

Output structured (e.g. JSON) logs in this file

This was already in place.

Store the error message with the ProjectRegistry table

Apart from the repository, I've also added the same for Wiki (in light of my next issue which will require more separation of these two).

Show a list of failed projects and their reasons for failures

This was discussed here and we won't have the UI for 10.3. But, we do need the API. I've added this based on this comment. However, we can consider not passing an ID altogether and just check the current node (a bit less RESTish, we'll need a separate endpoint). => I finally went for this.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan