RunTimeError on tearDownClass explained

The RunTimeError appearance on the console frequently caused
confusion, so it's message now explains the situation in a more
verbosely.

Change-Id: I904eeb09f6a7e7ea8a2121bc451d57ddf5f1d277
diff --git a/tempest/test.py b/tempest/test.py
index 8ce7af8..8bb5719 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -164,7 +164,11 @@
     if at_exit_set:
         raise RuntimeError("tearDownClass does not calls the super's "
                            "tearDownClass in these classes: "
-                           + str(at_exit_set))
+                           + str(at_exit_set) + "\n"
+                           "If you see the exception, with another "
+                           "exception please do not report this one!"
+                           "If you are changing tempest code, make sure you",
+                           "are calling the super class's tearDownClass!")
 
 atexit.register(validate_tearDownClass)