R1:14cc0abeb37f

Fix several safety issues with repository URIs

Summary:
See PHI234. Several issues here:

- The warning about observing a repository in Read/Write mode checks the raw I/O type, not the effective I/O type. That means we can fail to warn if other URIs are set to "Default", and "Default" is "Read/Write" in practice.
- There's just an actual typo which prevents the "Observe" version of this error from triggering properly.

Additionally, add more forceful warnings that "Observe" and "Mirror" mean that you…