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/
fix untentionally -> unintentionally in build_files.md

Reviewed at https://rbcommons.com/s/twitter/r/1430/
Avoid throwing in ApacheThriftGen.__init__

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/43046341

Bugs closed: 842

Reviewed at https://rbcommons.com/s/twitter/r/1428/