You will see the error in the 3d line of the BuildWrapper output - basically it can't load hunit. It should appear in bold.

I will point out I build buildwrapper and scion-browser from the command line with cabal, as letting eclipsefp to build them on startup always fails. I am on OS X Lion with the most recently released eclipsefp.

Also possibly related as sometimes an error pops up "Error rebuilding local database. Check the console log for more details".

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build-wrapper-json:[false,[{"s":"Error","t":"Could not find module `Test.HUnit.Base':\n It is a member of the hidden package **`HUnit-1.2.4.2'.\n Perhaps you need to add `HUnit' to the build-depends in your .cabal file.\n Use -v to see a list of the files searched for.","l":{"c":8,"l":22,"f":"src/Main.hs"}}]]
**