all packaging systems i've worked with are very strict about failing a build if any step in the build fails. drush_make seems to be almost the polar opposite of that at this point -- it plows on through just about everything to complete it's mission. ;)
for instance, you supply a release that doesn't exist for a package, drush_make just skips over it and continues, w/ no error
i would advise making it abort on any errors by default, and maybe having a --continue-on-errors flag which allows the current behavior if people really want it.