Invalid "While building Setup.hs" error message

[snoyberg]

I've noticed for a while now that error messages usually including a comment like:

While building custom Setup.hs for package...

This would make sense if the error occurred when building the Setup.hs for a custom build-type... but this message seems to occur regardless of build type, and regardless of where in the build things failed! This is just a minor cosmetic issue, but would be nice to fix up.