Fixed #25941 -- Improved error message for runtests.py when django is not on path.

tests/runtests.py

@@ -10,7 +10,13 @@ import sys
 import tempfile
 import warnings
 
+try:
     import django
+except ImportError as e:
+    raise RuntimeError(
+        'Django module not found, reference tests/README.rst for instructions.'
+    ) from e
+else:
     from django.apps import apps
     from django.conf import settings
     from django.db import connection, connections