KeyringLocked
when access to the keyring is denied (on get or set) instead
of PasswordSetError or KeyringError. Any API users
may need to account for this change, probably by catching
the parent KeyringError.
Additionally, the error message from the underying error is
now included in any errors that occur.keyring.core.init_backend, if any
limit function is supplied, it is saved and later honored by
the ChainerBackend as well.get_credentials, for backends
that can resolve both a username and password for a service.