[cmdline] Use cmdliner converter for regular result type.

src/main.ml

@@ -229,7 +229,7 @@ let verify_cmd =
   let term =
     let files =
       let doc = "File to verify." in
-      let file_or_stdin = Verification.File.(of_string, pretty) in
+      let file_or_stdin = Arg.conv' Verification.File.(of_string, pretty) in
       Arg.(non_empty & pos_all file_or_stdin [] & info [] ~doc ~docv:"FILE")
     in
     let prover =

src/verification.ml

@@ -48,10 +48,10 @@ module File = struct
 
   let of_string s =
     if String.equal s "-"
-    then `Ok Stdin
+    then Ok Stdin
     else if Caml.Sys.file_exists s
-    then `Ok (File s)
-    else `Error (Fmt.str "no '%s' file or directory" s)
+    then Ok (File s)
+    else Error (Fmt.str "no '%s' file or directory" s)
 
   let pretty fmt = function
     | Stdin -> Fmt.string fmt "-"

src/verification.mli

@@ -38,7 +38,7 @@ module File : sig
   type t
 
   val of_json_config : json_config -> t
-  val of_string : string -> [ `Ok of t | `Error of string ]
+  val of_string : string -> (t, string) result
   val pretty : Format.formatter -> t -> unit
 end