Fixed #27783 -- Switched VariableDoesNotExist.__str__() to repr() context.

Using __str__() and then repr'ing the result looks strange and can lead
to recursive rendering of forms.

tests/template_tests/test_base.py

@@ -0,0 +1,8 @@
+from django.template.base import VariableDoesNotExist
+from django.test import SimpleTestCase
+
+
+class VariableDoesNotExistTests(SimpleTestCase):
+    def test_str(self):
+        exc = VariableDoesNotExist(msg='Failed lookup in %r', params=({'foo': 'bar'},))
+        self.assertEqual(str(exc), "Failed lookup in {'foo': 'bar'}")