Report an error if an executable is not found, unify paths.
Report an error if an executable is not found, unify paths.

sdiehl/ghc,bgamari/shaking-up-ghc,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/shaking-up-ghc,snowleopard/hadrian,sdiehl/ghc,sdiehl/ghc