Add witness missing verdict to YAML witness validation

Better than "EXCEPTION (Failure)" as verdict.

Author: sim642

src/witness/yamlWitness.ml

@@ -565,6 +565,8 @@ struct
   }
 
   let validate () =
+    let open GobResult.Syntax in
+
     let location_locator = Locator.create () in
     let loop_locator = Locator.create () in
     (* TODO: add all CFG nodes, not just live ones from lh, like UnassumeAnalysis *)
@@ -579,9 +581,12 @@ struct
 
     let inv_parser = InvariantParser.create FileCfg.file in
 
-    let yaml = match Yaml_unix.of_file (Fpath.v (GobConfig.get_string "witness.yaml.validate")) with
-      | Ok yaml -> yaml
-      | Error (`Msg m) -> failwith ("Yaml_unix.of_file: " ^ m)
+    let* yaml =
+      Yaml_unix.of_file (Fpath.v (GobConfig.get_string "witness.yaml.validate"))
+      |> Result.map_error (fun (`Msg m) ->
+          Logs.error "Yaml_unix.of_file: %s" m;
+          "witness missing"
+        )
     in
     let yaml_entries = yaml |> GobYaml.list |> BatResult.get_ok in