Unhandled promise rejections

Sade is great, thanks!

I noticed that if my async handlers error out, then node exits with an UnhandledPromiseRejectionWarning but doesn't print the error message or stack trace or such. I think it'd be convenient if sade would catch these by default and print something, or give me an easy way to define an error handler (aside from manually wrapping each action).

No file, test, or entry point is named in the issue. Reproduce an async handler failure in a Sade CLI, then trace how rejected promises are handled; done should include a useful error message or stack trace by default, or a clearly documented error-handler path.

Written by the indexing model from the issue text.