fix: handle raft errors when unhealthy

[DanielArndt]

Removing Vault units while raft is in unhealthy state causes the charm to go into an unrecoverable error state. Handling these errors allows the charm to be removed gracefully.

TODO:

• Tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules