Confusion situation when linking ./dist/setup/setup is interrupted

(Imported from [Trac #665](http://hackage.haskell.org/trac/hackage/ticket/665), reported by @dcoutts on 2010-04-25)

If one accidentally interrupts cabal when it's linking ./dist/setup/setup then you can end up with the file existing it not being executable.

This leads to a confusing situation for the user. Running cabal install or cabal configure will fail silently. Running with -v indicates that something returns exit code 127. This corresponds to a permission error when trying to execute ./dist/setup/setup

Two improvements could be made:

• Report the failure to execute ./dist/setup/setup
  
• If ghc is interrupted when creating ./dist/setup/setup, cabal should delete the file, if it exists. That way, cabal will recompile on the next invocation.