Produce an error message when --inspection is set but the type [nonterminal]

cannot be generated because the type of some nonterminal symbol is unknown.

src/nonterminalType.ml

@@ -19,12 +19,13 @@ let tnonterminalgadtdata nt =
 (* This is the definition of the nonterminal GADT. Here, the data
    constructors have no value argument, but have a type index. *)
 
-exception MissingOCamlType
+exception MissingOCamlType of string
 
 let nonterminalgadtdef grammar =
   assert Settings.inspection;
   let comment, datadefs =
     try
+
       "The indexed type of nonterminal symbols.",
       List.fold_left (fun defs nt ->
         let index =
@@ -32,7 +33,7 @@ let nonterminalgadtdef grammar =
           | Some t ->
               TypTextual t
           | None ->
-              raise MissingOCamlType
+              raise (MissingOCamlType nt)
         in
         {
           dataname = tnonterminalgadtdata nt;
@@ -40,14 +41,28 @@ let nonterminalgadtdef grammar =
           datatypeparams = Some [ index ]
         } :: defs
       ) [] (nonterminals grammar)
-    with MissingOCamlType ->
+
+    with MissingOCamlType nt ->
       (* If the type of some nonterminal symbol is unknown, give up
          and define ['a nonterminal] as an abstract type. This is
          useful when we are in [--(raw)-depend] mode and we do not
          wish to fail. Instead, we produce a mock [.mli] file that
-         is an approximation of the real [.mli] file. *)
+         is an approximation of the real [.mli] file. When we are
+         not in [--(raw)-depend] mode, though, this is a problem.
+         We display an error message and stop. *)
+      match Settings.depend with
+      | Settings.OMRaw
+      | Settings.OMPostprocess ->
           "The indexed type of nonterminal symbols (mock!).",
           []
+      | Settings.OMNone ->
+          Error.error [] (Printf.sprintf "\
+            the type of the nonterminal symbol %s is unknown.\n\
+            When --inspection is set, the type of every nonterminal symbol must be known.\n\
+            Please use --infer or specify the type of every symbol via %%type declarations."
+            nt
+          )
+
   in
   [
     IIComment comment;