Ending notebook execution early with sys.exit without raising an exception

[yusuphisms]

Hi!

I'm a newbie around these parts (and contributing to open source in general!) but thought I might make an attempt to resolve #435 . Noticed that without arguments to exit() or with None, evalue returns an empty string; that should also be evaluated the same as 0 I imagine? Any and all feedback is welcome 🙏🏾

Thank you!
Yusuph

[MSeal]

The change looks good, we just need a couple unittests to cover the behavior. Probably one unittest each with an exit(), exit(0), and finally an exit(1) added to test_execute.py. Don't worry about the build failing in travis / azure, there's an issue with azure blob dependency causing it to fail atm.

[yusuphisms]

Thanks for the review! Added tests -- tried to follow convention but let me know if I need to make any changes.

Also, this is strange but when I was running my tests, I occasionally got this in the output: Timeout waiting for IOPub output . It would cause that test to fail. When that did not happen, the tests would pass. Not sure if that's related to the new tests or not.

[MSeal]

LGTM -- the test failure you saw is a known issue in ipython versions for python 2. Either your dependencies were old or you were testing in python 2, which we're going to drop direct support for at the end of the year.