fix(cli): catch OAuth error in register-webapp

[Caushi]

Summary

• bfabric-cli auth register-webapp crashed with a raw Python traceback when the OAuth session's refresh token was expired/revoked, instead of printing a clean error like the rest of the command.
• client.auth.password.get_secret_value() (the call that triggers a token refresh) sat outside any try/except, so the BfabricOAuthError raised by credential_provider.py had no local handler and propagated past __main__.py's bare app() call.
• Wrapped that call in the same try/except pattern already used by its neighbors in this file, so it now prints Error: ... and exits 1.

Test plan

• Manually confirmed the code path: credential_provider._ensure_token raises BfabricOAuthError (a RuntimeError) on invalid_grant; the new try/except in register_webapp.py catches it and prints a clean message instead of a traceback.
• Reproduce locally with a stale/revoked refresh token cached for an OAuth env and confirm bfabric-cli auth register-webapp now prints Error: OAuth session expired (...) and exits 1 instead of a traceback.

[leoschwarz]

Thank you. I think error handling in the CLI is a bit weak in general, maybe at some point making use of Cyclopts error handling https://cyclopts.readthedocs.io/en/latest/app_calling.html#exception-handling-and-exiting would make sense.