Fix error message when isolated tenant subnet creation fails

The current code assumes that setting the message of an exception will
cause the desired message to be displayed but this does not appear to be true.

Change-Id: I7e91179e7cb426645e9250671d3b54475f79a613
diff --git a/tempest/common/isolated_creds.py b/tempest/common/isolated_creds.py
index 228e47c..1ce1e39 100644
--- a/tempest/common/isolated_creds.py
+++ b/tempest/common/isolated_creds.py
@@ -203,9 +203,8 @@
                 if 'overlaps with another subnet' not in str(e):
                     raise
         else:
-            e = exceptions.BuildErrorException()
-            e.message = 'Available CIDR for subnet creation could not be found'
-            raise e
+            message = 'Available CIDR for subnet creation could not be found'
+            raise Exception(message)
         return resp_body['subnet']
 
     def _create_router(self, router_name, tenant_id):