cli: guess returns exit code 0 with empty stdout if no matches found

[Ifropc]

If no matches are found (given a valid base64 string but an invalid XDR), guess command prints nothing and returns code 0, which might be confusing to the user.

We can return an error if no matches are found, or stderr a warning with code 0

(Personally I prefer the second approach)

[Ifropc]

@leighmcculloch would be happy to hear your thought on that, should be a small change either way 🤔