patch applied (ghc-6.8/ghc): MERGED: Print better error message for reading External Core

[Ian Lynagh]

Sun Feb 24 07:39:36 PST 2008  Ian Lynagh <igloo at earth.li>
  * MERGED: Print better error message for reading External Core
  Tim Chevalier <chevalier at alum.wellesley.edu>**20080217223844
   
   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 -29 +2