Add calls that return exit-code so that errors can be handled by caller

[Wojtek242]

Continuing from my previous pull request which I had to close since it required a function that was only introduced in Emacs 25.

I would like to add some additional functions that return normally when an error is encountered so that the caller can handle the error separately. This is done by additionally returning the error code.

[kiwanami]

Thank you. It's great code! I'll check later.