This adds "publishing" signals, executed before any data is saved.  The intention is that an extension can block publishing if, for example, there's a problem communicating with an outside system, by raising an exception.
I added a PublishError class and made web API publish functions turn them into WebAPIErrors.  This is the part I'm least certain about.  I want to allow extension to define the errors, but it doesn't make sense for the publishing signal handlers to be throwing WebAPIErrors directly.  The compromise I came up with was to just put the PublishError string into a generic WebAPIError with a special user code of 999.  Other ideas welcome; I just wanted something to start a conversation.
The ReviewRequestEditor model already had some basic error handling (a simple alert dialog).  I added similar error handling to DraftReviewBannerView and ReviewReplyDraftBannerView.  As far as I can tell, that covers all interfaces to updating (publishing) reviews, review requests, and review replies.

All tests pass except some VCS-specific ones, which were skipped.  I also did some manual verification of the full system by throwing PublishError exceptions in an extension and verifying that the message appeared in an alert box in the UI.