Fix error message in scrooge_gen

I hit this error within an error when I tried to run the gen goal.

Testing Done:
Built a pex, triggered the error.

Reviewed at https://rbcommons.com/s/twitter/r/1426/
Sync of https://github.com/pantsbuild/pants/commit/9a1cf7ab9ee1662c4d1a69c5579ce1935da24a99