Update exception handling section to cover throwing a promise

Seems like we're getting close on #65, so I wanted to take a stab at this to see how it reads.  I tried to parallel the returnedPromise section for the most part.  I think there's more reorganization needed, like grouping the returned value and returned promise bullets together, or at least placing them one after another, and maybe a few other tweaks.  That could be a separate pull request, or could be integrated here.  Either way, it may require some similar reordering in the test suite.