[#2167] Remove ActionError.extra_msg.

See: https://github.com/ckan/ckan/issues/2167

The key to this is `ActionError.__init__` which clearly only allows one message, and stores it in self.extra_msg, so self.message is always blank, making it a bit stupid to have the complexity in __str__/__unicode__ functions.

NB This PR is not to merge to master, it is to merge to another branch (2165). This is because 2165 highlights issues with unicode and adds a couple of extra tests that exercise NotFound, showing this patch works.