settings.py: additional checks for ImportError when importing local_settings

In its current state, settings.py catches and ignores all ImportErrors when loading the local settings. While this behavior was intended to deal with a missing local_settings.py, it also hides ImportErrors caused by import
statements *inside* local_settings.py, e.g.  for setting up deployment-specific ldap authentication.

This simple fix will rethrow any ImportErrors that don't have "local_settings" in their message.