adding lang string for 'no_right_issuers'

[individual-it]

When there is no valid issuer the propper error message needs to be shown, but the language string was not defined.

The string is used in https://github.com/owncloud/moodle-repository_ocis/blob/c91975267f4bd00700266eb272de6db588cdadfd/lib.php#L269