Best effort error handling when cloud code returns unexpected data

[cheungpat]

Coding error can cause cloud code runtime to return unexpected data. Instead of returning a parse error to the user, the error object could provide more information.

Currently skygear-server returns this:

{"error":{"name":"UnexpectedError","code":10000,"message":"failed to parse response: json: cannot unmarshal string into Go value of type common.ExecError"}}