NoExecutableFound should write to stderr and return non-zero

[uranusjr]

I was trying to use something like py -X.Y -c "import sys; print(sys.executable, end='')" to detect the location of a given interpreter, but the execution result for a non-existant interpreter makes it difficult to handle the output. It would be much easier if the launcher fails in a more script-friendly way.

[brettcannon]

Yep, hence why both issues are listed in the TODO required to reach an MVP. 😉

[brettcannon]

OK, everything that was writing to stdout on failure should now be writing to stderr and there is a non-zero exit code as well. Let me know if it works for you!

[uranusjr]

It works flawlessly 🎉