Incorrect error format

[cheungpat]

If an error occurred with js cloud code, the skygear-node returns an error like this:

{"error":"error message"}

skygear-server expects the data in common.ExecError format:

{"error":{"name":"<error type>","code":10000,"message":"<error message>"}}

referencing issue at SkygearIO/skygear-server#232