More Information on a failing test.

dav_events.tests: sometimes an OneclickActionTest failes without
providing enough information.

dav_events/tests/test_oneclickactions.py

@@ -121,7 +121,11 @@ class ActionTestCase(EmailTestMixin, RoleMixin, EventMixin, TestCase):
                        'user': user.get_full_name(),
                    })
         html = message.replace('\'', ''')
-        self.assertInHTML(html, content)
+        # Sometimes this test fail, and we cannot see the tested content, so we create our own Exception
+        try:
+            self.assertInHTML(html, content)
+        except AssertionError as e:
+            raise AssertionError('Not in HTML:\n{}\n-----\n{}\n'.format(html, content)) from e
         self.assertRegex(content, r'alert-success')
 
     def setUp(self):