--- a/testing/web-platform/tests/webdriver/tests/support/asserts.py
+++ b/testing/web-platform/tests/webdriver/tests/support/asserts.py
@@ -63,45 +63,49 @@ def assert_error(response, error_code):
     :param error_code: string value of the expected "error code"
     """
     assert response.status == errors[error_code]
     assert "value" in response.body
     assert response.body["value"]["error"] == error_code
     assert isinstance(response.body["value"]["message"], basestring)
     assert isinstance(response.body["value"]["stacktrace"], basestring)
 
+
 def assert_success(response, value=None):
     """Verify that the provided wdclient.Response instance described a valid
     error response as defined by `dfn-send-an-error` and the provided error
     code.
 
     :param response: wdclient.Response instance.
     :param value: Expected value of the response body, if any.
 
     """
-    assert response.status == 200
+    assert response.status == 200, str(response.error)
+
     if value is not None:
         assert response.body["value"] == value
     return response.body.get("value")
 
+
 def assert_dialog_handled(session, expected_text):
     result = session.transport.send("GET",
                                     "session/%s/alert/text" % session.session_id)
 
     # If there were any existing dialogs prior to the creation of this
     # fixture's dialog, then the "Get Alert Text" command will return
     # successfully. In that case, the text must be different than that
     # of this fixture's dialog.
     try:
         assert_error(result, "no such alert")
     except:
         assert (result.status == 200 and
                 result.body["value"] != expected_text), (
                "Dialog with text '%s' was not handled." % expected_text)
 
+
 def assert_same_element(session, a, b):
     """Verify that two element references describe the same element."""
     assert isinstance(a, dict), "Actual value is not a dictionary"
     assert isinstance(b, dict), "Expected value is not a dictionary"
     assert element_key in a, "Actual value does not describe an element"
     assert element_key in b, "Expected value does not describe an element"
 
     if a[element_key] == b[element_key]: