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It fails to copy the rpm there (obviously as the dir doesn't exist), and does give an error but it is annoying because I left a rather large rpm building overnight and it failed. rpm should check for the destination directory it will dump the newly created rpm into and warn if it doesn't exist BEFORE doing all the compiling. Minor but annoying.
There's nothing stopping you from writing your specfile as %build [ ! -d %{_rpmdir}/%{_arch} ] && exit 1 if you wish this functionality. There's a limit on what rpm can do without intelligent intervention.