Do not delay or set to an error if an exception is raised in dequeueMessage, because invoke or invokeGroup should never emit an exception, so an exception should be nothing with a message itself.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@13713 20353a03-c40f-0410-a6d1-a30d3c3de9de