improve error messages to reflect compressed and encrypted files

- Move "no files found" error message into ScanArchive().
  This avoids a redundant error message, if ScanArchive() was ended
  early, because of a damaged / completely encrypted / ... archive.
- Keep track of the number of encrypted and compressed files, and,
  if no usable media file was found, show error messages to indicate
  thse compressed or unusable files, if present. In this case the
  encrypted files message gets a higher priority than the compressed
  message.