patch applied (ghc): Print better error message for reading External Core

[Tim Chevalier]

Sun Feb 17 14:38:44 PST 2008  Tim Chevalier <chevalier at alum.wellesley.edu>
  * Print better error message for reading External Core
  
  GHC panicked with a "Prelude.undefined" error message if you tried to
  compile a .hcr file. Since support for reading ExternalCore simply does
  not exist, I added an error message to say that.
  
  Please merge to 6.8. Thanks.
  

    M ./compiler/main/HscMain.lhs -34 +3