Do not return a unclear error message if json_data is not set

Test with :
   curl -d ''  http://127.0.0.1:8080/broker/cartlist

While this should return a error message, the current code return
a ruby traceback, which is not helpful to understand what is going on.
Merge pull request #500 from mscherer/fix_missing_jsondata

Merged by openshift-bot