Show proper error message if org is SAML enabled when try to login directly to Split

If the Split organization has SAML option enabled to integrate with SAML providers like OKTA, users cannot login directly to app.split.io login page.

There is no clear error message though in app.split.io login page, it will just say "login failed".

Would be useful to show "The organization has SAML enabled, please login through your SAML provider.