handle NOTMUCH_STATUS_ERROR_MESSAGE

[David Bremner]

There's what looks like a race condition discussed at

	https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=698810

This doesn't do anything about the race condition, but tries to report
more helpful information when it happens.

If I knew how to test this properly, I'd probably know how to
reproduce the race condition.

The first page in the series is needed for some other things, so it's
not as much overkill as it looks.