Write to error log if a group or user mapping is defined but mapping failed

[Sebobo]

In one of my projects the customer changed something in their LDAP and the users
couldn't get mapped anymore which led to the login form just to reload and not log in without
any helpful logging. The basic authentication works out but the user cannot do anything.

I would add some error logging to the LdapProvider in case this happens so at least this way it's possible to find the issue faster.

[Sebobo]

Additionally the docs should be updated on how to handle this case for the users view.