Report parse errors with old opam-file-format

The sentinel group of kind `#` is now processed correctly in
OpamFile.SyntaxFile. This allows, for example, opam 2.2 to use
opam-file-format 2.1 and still have `opam-version: "2.2"`.

src/format/opamFile.ml

@@ -1016,15 +1016,24 @@ module SyntaxFile(X: SyntaxFileArg) : IO_FILE with type t := X.t = struct
   module IO = struct
     let to_opamfile filename t = Pp.print X.pp (filename, t)
 
+    let catch_future_syntax_error = function
+    | {file_contents = [{pelem = Variable({pelem = "opam-version"; _}, {pelem = String ver; _}); _ };
+                        {pelem = Section {section_kind = {pelem = "#"; _}; _}; pos}]; _}
+      when OpamVersion.(compare (nopatch (of_string ver)) (nopatch X.format_version)) <= 0 ->
+        raise (OpamPp.Bad_format (Some pos, "Parse error"))
+    | opamfile -> opamfile
+
     let of_channel filename (ic:in_channel) =
-      Pp.parse X.pp ~pos:(pos_file filename) (Syntax.of_channel filename ic)
+      let opamfile = Syntax.of_channel filename ic |> catch_future_syntax_error in
+      Pp.parse X.pp ~pos:(pos_file filename) opamfile
       |> snd
 
     let to_channel filename oc t =
       Syntax.to_channel filename oc (to_opamfile filename t)
 
     let of_string (filename:filename) str =
-      Pp.parse X.pp ~pos:(pos_file filename) (Syntax.of_string filename str)
+      let opamfile = Syntax.of_string filename str |> catch_future_syntax_error in
+      Pp.parse X.pp ~pos:(pos_file filename) opamfile
       |> snd
 
     let to_string filename t =