change teardown check to LOG.error

even with the warning message, this is completely confusing for
people when it shows up in the console logs. Instead lets just
log it at error level.

Change-Id: Iaa3a9d337adcfde9ddf702f8cfab1b9dc74c6559
diff --git a/tempest/test.py b/tempest/test.py
index 2125047..58842e4 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -199,13 +199,11 @@
 
 def validate_tearDownClass():
     if at_exit_set:
-        raise RuntimeError("tearDownClass does not call the super's "
-                           "tearDownClass in these classes: "
-                           + 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!")
+        LOG.error(
+            "tearDownClass does not call the super's "
+            "tearDownClass in these classes: \n"
+            + str(at_exit_set))
+
 
 atexit.register(validate_tearDownClass)