fusefrontend: warn about missing diriv

The comment is outdated, at this point, we should
really not get any errors from ReadDirIVAt.

The change is best seen when running the fsck tests. Before:

  fsck: error opening dir "missing_diriv": 2=no such file or directory

After:

  OpenDir "K2m0E6qzIfoLkVZJanoUiQ": could not read gocryptfs.diriv: no such file or directory
  fsck: error opening dir "missing_diriv": 5=input/output error

See https://github.com/rfjakob/gocryptfs/issues/403 , where
the extra info would have been helpful.

internal/fusefrontend/fs_dir.go

@@ -288,11 +288,6 @@ func (fs *FS) OpenDir(dirName string, context *fuse.Context) ([]fuse.DirEntry, f
 		// Read the DirIV from disk
 		cachedIV, err = nametransform.ReadDirIVAt(fd)
 		if err != nil {
-			// The directory itself does not exist
-			if err == syscall.ENOENT {
-				return nil, fuse.ENOENT
-			}
-			// Any other problem warrants an error message
 			tlog.Warn.Printf("OpenDir %q: could not read %s: %v", cDirName, nametransform.DirIVFilename, err)
 			return nil, fuse.EIO
 		}