I would like to know what our resident Windows users think about this (Paul, Steve, Zach).

Reading the above arguments, I'd be inclined to settle on 15 (that is, the non-negative "signal" number).  While it is not consistent with what "taskkill" or other APIs do, it makes it clear that the process was terminated in a certain way.  Certainly, there is a slight chance that 15 is a legitimate error code returned by the process, but that is far less likely than returning 1 as a legitimate error code, which I presume is extremely common.

In any case, this can't go in a bugfix release, so marking as 3.7-only.