set_passwd and expire_password reject invalid "protocol" with "Invalid
parameter 'protocol'".  Misleading; the parameter is valid, its value
isn't.  Improve to "Parameter 'protocol' expects 'vnc' or 'spice'".

expire_password fails with "Could not set password".  Misleading;
improve to "Could not set password expire time".

QERR_SET_PASSWD_FAILED is now unused.  Drop.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113082626.2725812-5-armbru@redhat.com>