give exception a nice message (closes #22379)

Patch by Yongzhi Pan.
diff --git a/Lib/test/test_userstring.py b/Lib/test/test_userstring.py
index 51d8e8b..9cca14a 100644
--- a/Lib/test/test_userstring.py
+++ b/Lib/test/test_userstring.py
@@ -28,14 +28,12 @@
             realresult
         )
 
-    def checkraises(self, exc, object, methodname, *args):
-        object = self.fixtype(object)
+    def checkraises(self, exc, obj, methodname, *args):
+        obj = self.fixtype(obj)
         # we don't fix the arguments, because UserString can't cope with it
-        self.assertRaises(
-            exc,
-            getattr(object, methodname),
-            *args
-        )
+        with self.assertRaises(exc) as cm:
+            getattr(obj, methodname)(*args)
+        self.assertNotEqual(cm.exception.message, '')
 
     def checkcall(self, object, methodname, *args):
         object = self.fixtype(object)