You may consider making the error message here more verbose and maybe honor --keep-going.
%% emerge -e world --keep-going
Calculating dependencies... done!
>>> Verifying ebuild manifests
>>> Starting parallel fetch
>>> Emerging (1 of 77) sys-apps/portage-2.2.00.15027
Permission denied: 'makedirs('/binpkgs/sys-apps')'
%% echo $?
Another option would be to check perms of $PKGDIR earlier in the process..?