test_classes: Refine assert_json_success output with exception chaining.

Now a failure from assert_json_success and friends shows the full
server-side traceback from the JsonableError that caused the failure,
not just the test-side traceback for `AssertionError: 400 != 200 :
{message}`.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/middleware.py

@@ -392,7 +392,9 @@ class JsonErrorHandler(MiddlewareMixin):
                 # just return the response without logging further.
                 return response
         elif RequestNotes.get_notes(request).error_format == "JSON" and not settings.TEST_SUITE:
-            response = json_response(res_type="error", msg=_("Internal server error"), status=500)
+            response = json_response(
+                res_type="error", msg=_("Internal server error"), status=500, exception=exception
+            )
         else:
             return None