Throw error if an unregistered datasource is used in a command.

[samyakkkk]

Using a datasource without having it registered would lead to it not being used and would confuse users who wouldn't see the expected results.

Add a check for the same during publishing and agent.

[zexross]

@samyakkkk I'm taking this issue and putting up a deadline of Thursday for this. Hope that's alright!