What is the meaning of this Upstart init error?

$ start playframework
start: Rejected send message, 1 matched rules; type="method_call", 
sender=":1.6" (uid=1000 pid=1080 comm="start portal ") 
interface="com.ubuntu.Upstart0_6.Job" member="Start" error name="(unset)" 
requested_reply="0" destination="com.ubuntu.Upstart"
(uid=0 pid=1 comm="/sbin/init")

I'm trying to get playframework to start using this Upstart script.

I saw similar errors posted around the web, and here, but never found a general explaination of what they mean. Does it mean the process that's being started returned an error? What's the general way to debug these kinds of errors?

It means you don't have permissions to tell Upstart what to do. As you said, you're missing sudo and normal user has no permissions to start or stop system jobs. The cryptic string is just very engineerish (and exact) way to say it :-)