Wibble cabal-bin's error message

We don't need to put the program name in it, as that happens automatically

libraries/cabal-bin.hs

@@ -34,5 +34,5 @@ main = do
             _ | packageName pd == PackageName "Cabal" ->
                               -- Cabal is special...*sigh*
                               Simple.defaultMainArgs                     args
-              | otherwise  -> die "cabal: Don't know what to do!"
+              | otherwise  -> die "Don't know what to do!"