Improve handling connections errors to LDAP server #3
Description
For now the plugin throws an error since the ldap provider instance is null in case there's an error when connecting to the ldap server.
It should:
- throw an exception in case the connection fails
- catch the exception on login so that the default yii2-usuario login can continue
- throw an exception in all the other cases in the "before" events (even if the actual procedure is done in the "after" one)