Don't crash if Django isn't installed

DjangoInstalledChecker will catch the condition and inform the user,
but we should not crash before that checker gets executed.

pylint_django/checkers/foreign_key_strings.py

@@ -78,9 +78,12 @@ def open(self):
         # must wait until some module is inspected to be able to raise... so that
         # state is stashed in this property.
 
-        from django.core.exceptions import (  # pylint: disable=import-outside-toplevel
-            ImproperlyConfigured,
-        )
+        try:
+            from django.core.exceptions import (  # pylint: disable=import-outside-toplevel
+                ImproperlyConfigured,
+            )
+        except ModuleNotFoundError:
+            return
 
         try:
             import django  # pylint: disable=import-outside-toplevel