Make URLFormatter.parse_return raise InvalidAuth when fields are missing

This way, errors in AUTHGROUPEX_FIELDS configuration are no longer
reported as "Received a AuthResult object without a username.", which is
what the authentification backend reports when processing unsuccessful
AuthResult objects.
Remove AuthResult.success field

This field was always true, as an unsuccessful authentication raises
InvalidAuth exception.
Fix typo in AuthGroupeXConf: s/config_/configure_/
Add "coverage" target to Makefile