Return explicit error if no token endpoint auth method is set
trying to create an OIDC link between 2 LLNG, I've received this :
[Wed Sep 07 19:40:56.090980 2016] [:error] [pid 519:tid 140107823253248]
Can't call method "is_error" on an undefined value at
/usr/share/perl5/Lemonldap/NG/Portal/_OpenIDConnect.pm line 441.
It means that $auth_method isn't "client_secret_basic" neither "client_secret_post"