Geo: improve error message when clone_url_prefix is nil

What does this MR do?

If primary node doesn't contain a clone_url_prefix, replication will fail with an error pointing to that instead of a vague fatal: Could not read from remote repository.

Are there points in the code the reviewer needs to double check?

Not 100% sure behavior of existing sync workers wether we should execute after hooks in case of any exception or not.

Why was this MR needed?

The current error is misleading and fixing the issue is easier when you know where it's coming from.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2896 (closed)