I'd assumed that the backend being killed should be able to get out an
error message - but it turns out it's not guaranteed that it's not
still sending a ready-for-query.  Really need to do something about
getting these error message to the client.

Reported-By: Thomas Munro, Tom Lane
Discussion:
	https://postgr.es/m/CAEepm=0TE90nded+bNthP45_PEvGAAr=3gxhHJObL4xmOLtX0w@mail.gmail.com
	https://postgr.es/m/14968.1506101414@sss.pgh.pa.us