Merge pull request #8224 from w3c/sync_3f6bfc6fad9bac3e5ba732939e70140057e8760b

Display error when response is unsuccessful.
diff --git a/webdriver/tests/support/asserts.py b/webdriver/tests/support/asserts.py
index 21cc07b..c2cf84a 100644
--- a/webdriver/tests/support/asserts.py
+++ b/webdriver/tests/support/asserts.py
@@ -68,6 +68,7 @@
     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
@@ -77,11 +78,13 @@
     :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)
@@ -97,6 +100,7 @@
                 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"