fix(cli): report campus.audit credential-store failures instead of a traceback #57

Context

The campus audit group (#55) reads and writes the audit API key in the
shared credential store, which is keyring-backed with a file fallback. A
store that cannot be read or written (locked keyring, unreadable
credentials.json) raises CredentialError.

campus audit logout catches it and reports it cleanly, but the other two
paths do not:

• campus audit login — credentials.set_audit_api_key(...) is
  unguarded, so a store failure escapes as a full Python traceback.
• get_audit_client() — credentials.get_audit_api_key() (and the URL
  binding read) are unguarded, so every API-backed audit command raises a
  traceback instead of an error message.

The rest of the CLI already treats a credential-store failure as a clean,
reported error; the audit group should match.

Proposed fix

Wrap the credential-store reads and writes in the audit group the same way
campus audit logout already does: catch CredentialError, print an
actionable error with print_error, and exit 1.

Acceptance

• campus audit login with a store that raises CredentialError exits
  1 with a message naming the failure, and no traceback.
• An API-backed audit command with a store that raises
  CredentialError exits 1 with a message naming the failure, and no
  traceback.
• The stored audit key's URL binding read is covered by the same guard.