It shouldn't be fatal if SSTATE_MIRROR returns 404 for that file, but if it's available, then it would be usefull to download it. This way user can compare .siginfo files when he sees that next build is not reusing something from remote SSTATE_MIRROR but creates new.
It looks pretty straight forward to just add a couple lines to sstate.bbclass to fetch the .siginfo, and it will fail quietly
http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=97a91e81eaa4a893582161a13465775183669965
Thanks!