Fixed #23556 -- Raised a more meaningful error message when migrations refer to an unavailable node

tests/migrations/test_migrations_squashed_complex/1_auto.py

@@ -0,0 +1,11 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    operations = [
+        migrations.RunPython(lambda apps, schema_editor: None)
+    ]