Distfile for 20170202 version does not exist anymore.
Do you mean x11-wm/stumpwm-contrib?
Yes, x11-wm/stumpwm-contrib, shame on me.
Thanks for the report. Version bumped to last date (stumpwm-contrib-20170314) and SRC_URI moved. It should install fine now. https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=74c9625d93605251ae4670ae170ff7131b460e03