Encourage use of Future.exception over throw for non-fatals.

This PR supersedes Moses' earlier one which addresses these points. I've taken Marius' changes into consideration, modulo some formatting and minor wording cleanups.
s/which/that/.

Also, more direct wording, and kill the double negative (it's confusing). To simplify, I'd say:

  "Do not throw your own exceptions in methods that return futures."

  "Do not throw fatal exceptions in methods that return futures."