Term.exit and exit_status_of_result require a unit result

[talex5]

Previously, exit allowed passing any value to it and just ignored it. This leads to various errors going undetected. e.g. this program returns a successful exit status and displays no output, which is surprising:

let revolt () = Error (`Msg "Revolt!")
open Cmdliner
let revolt_t = Term.(const revolt $ const ())
let () = Term.exit @@ Term.eval (revolt_t, Term.info "revolt")

It also causes confusion when an argument is missing, e.g.

let chorus count msg =
  for i = 1 to count do print_endline msg done

open Cmdliner

let count =
  let doc = "Repeat the message $(docv) times." in
  Arg.(value & opt int 10 & info ["c"; "count"] ~docv:"COUNT" ~doc)

let chorus_t = Term.(const chorus $ count)

let () = Term.exit @@ Term.eval (chorus_t, Term.info "count")

This program compiled without error, but produces no output.

With this change, both programs report errors at compile-time.

[talex5]

Testing this with the packages I have installed in my switch at the moment:

• alcotest, crowbar and opam-0install were previously correct but needed minor updates for this change.
• hxd and duff.0.2 passed errors to cmdliner expecting them to be reported. This PR would have prevented these bugs (duff removed the executables in 0.3).
• ocurrent and ocaml-ci were incorrectly passing errors to cmdliner, but also logged them (presumably because I'd been confused by this behaviour in the past).

[dbuenzli]

I wish I had done these right from the onset :-( I'm a bit nervous changing these things. But your bad examples are compelling enough. Your patch is in as 2f02ae4. Thanks !

[dbuenzli]

@talex5 you also found a bug in one of my programs which was using Term.exit instead of Term.exit_status :-) Thanks.