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.
Added Andreas Fleig to AUTHORS.
Fix local_settings import error check for Python 3.