fusefrontend: warn about missing diriv
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The comment is outdated, at this point, we should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.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs