Should gracefully handle script that is not a Dagger pipeline

[jpadams]

          > ## Another varation that still does not work (2)

this seemed to work, it's the docker stop part that failed after running the pipeline since it seems like the dagger engine wasn't started. @alex9fredericks could it be possible that the python main.py is not currently running a dagger pipeline in this case?

In any case we should handle this gracefully also.

Originally posted by @marcosnils in https://github.com/dagger/dagger-for-github/issues/106#issuecomment-1872178857