Ticket 176: Fix verbosity error to include the valid -v values

Fixed:
http://hackage.haskell.org/trac/hackage/ticket/176

Distribution/Verbosity.hs

@@ -98,7 +98,8 @@ flagToVerbosity (Just s)
        [(i, "")] ->
            case intToVerbosity i of
                Just v -> v
-               Nothing -> error ("Bad verbosity " ++ show i)
+               Nothing -> error ("Bad verbosity: " ++ show i ++
+                                 ". Valid values are 0..3")
        _ -> error ("Can't parse verbosity " ++ s)
 
 showForCabal, showForGHC :: Verbosity -> String