Cope better with `EDITOR` not being defined

There are a couple of ways this could be achieved:

1. Fail-fast with a nice error message if EDITOR has not been defined.
2. Ask the developer to enter the path to their editor in lieu of EDITOR not being defined.

Ideally, my preference would be for option 2, as I plan to use publish-release within the postpublish hook of all our modules, and this would allow developers who run npm publish to still have things work, rather than requiring them to define EDITOR, then re-run npm run postpublish.