Net.lookup returns Or_error rather than raising

[rgrinberg]

I don't think Net.lookup can throw any other way so Or_error is appropriate.

[dbuenzli]

Don't you want to be able to distinguish the errors programatically ? If that's the case you should rather use a proper variant for the error report (sorry don't have time to have a close look at that).

[rgrinberg]

@dbuenzli In this case we don't care about distinguishing what particular error it is because the function itself isn't even exported. Also, I agree with using variants for representing errors but it seems like it's core's convention to use Error.t.

I'll fix the message however.

https://realworldocaml.org/v1/en/html/error-handling.html