getTransactionalVariable has always been implemented to return a global
cache object (1 per thread): old implementation used the given context only to
have access to the portal.

So we enforce this with a warning to make sure people don't expect retrieving
1 cache object per given context.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@38366 20353a03-c40f-0410-a6d1-a30d3c3de9de