Don't report errors on cleaning when it's already clean

We now ignore file not found errors when deleting the packages dir.
Otherwise when we clean and then clean again we get an exception since
the directory we're trying to delete has already been deleted.

cabal-install/Hackage/Clean.hs

@@ -15,11 +15,15 @@ module Hackage.Clean
     ) where
 
 import Hackage.Types (ConfigFlags(..))
+import Hackage.Utils (fileNotFoundExceptions)
 
 import System.Directory (removeDirectoryRecursive)
+import Control.Exception (catchJust)
 
 -- | 'clean' removes all downloaded packages from the {config-dir}\/packages\/ directory.
 clean :: ConfigFlags -> IO ()
 clean cfg
-    = removeDirectoryRecursive (configCacheDir cfg)
-
+    = catchJust fileNotFoundExceptions
+        (removeDirectoryRecursive (configCacheDir cfg))
+	-- The packages dir may not exist if it's already cleaned:
+	(const (return ()))