Fix airflowctl auth login reporting success when keyring backend is unavailable

[dheerajturaga]

Credentials.save() caught NoKeyringError and logged it without re-raising, so the login command never saw the failure and printed "Login successful!" despite the token never being persisted.

2026-01-31 17:14:06 [error    ] No keyring backend available: No recommended backend was available. Install a recommended 3rd party backend package; or, install the keyrings.alt package if you want to use the non-recommended backends. See https://pypi.org/project/keyring for details. [airflowctl.api.client]
2026-01-31 17:14:06 [error    ] No recommended backend was available. Install a recommended 3rd party backend package; or, install the keyrings.alt package if you want to use the non-recommended backends. See https://pypi.org/project/keyring for details. [airflowctl.api.client]
2026-01-31 17:14:06 [error    ] No recommended backend was available. Install a recommended 3rd party backend package; or, install the keyrings.alt package if you want to use the non-recommended backends. See https://pypi.org/project/keyring for details. [airflowctl.api.client]
Login successful! Welcome to airflowctl

[bugraoz93]

Great one! Thanks!