error message printed to stderr, exit with error code on failure

[WnP]

Features:

• error messages are now printed to stderr
• when program failed it return an error code

error code use case:

stackit -e "python broken_app.py" && vim broken_app.py

if no error question is return by stackit vim is not launched

if questions are return I can found a solution on StackOverflow and fix my broken_app.py using vim

[lukasschwab]

@WnP I like this, and it should hopefully be an easy pull request to evaluate.

Unfortunately, I'm super swamped right now, hence the delay––I'll try to review this by EOD, but it might take until tomorrow.

[lukasschwab]

Works beautifully––merging!