avoid untrappable exception if dirContentsRecursive is run on a directory

that doesn't exist, or cannot be read

The problem is its use of unsafeInterleaveIO, which causes its IO code
to run when the thunk is forced, outside any exception trapping the caller
may do.

Utility/Directory.hs

@@ -35,14 +35,15 @@ dirContents :: FilePath -> IO [FilePath]
 dirContents d = map (d </>) . filter (not . dirCruft) <$> getDirectoryContents d
 
 {- Gets files in a directory, and then its subdirectories, recursively,
- - and lazily. -}
+ - and lazily. If the directory does not exist, no exception is thrown,
+ - instead, [] is returned. -}
 dirContentsRecursive :: FilePath -> IO [FilePath]
 dirContentsRecursive topdir = dirContentsRecursive' topdir [""]
 
 dirContentsRecursive' :: FilePath -> [FilePath] -> IO [FilePath]
 dirContentsRecursive' _ [] = return []
 dirContentsRecursive' topdir (dir:dirs) = unsafeInterleaveIO $ do
-	(files, dirs') <- collect [] [] =<< dirContents (topdir </> dir)
+	(files, dirs') <- collect [] [] =<< catchDefaultIO (dirContents (topdir </> dir)) []
 	files' <- dirContentsRecursive' topdir (dirs' ++ dirs)
 	return (files ++ files')
 	where