Show more friendly message on RBAC authorization errors

Currently, when logged in as a read-only user, the UI still shows operations that require read-write. When the user attempts them, there is a cryptic message about an API failure.

As the first step we should catch all RBAC-related authorization failures and display a more user-friendly error message for the cause of these (Your account does not have sufficient permissions to carry out this action... etc.).

Simultaneously Michael Clelland will review and disable actions not allowed for a given user (e.g. grey out buttons).

Acceptance criteria

• More user-friendly error message when encountering RBAC authorization failures