Apply suggestions from code review

Co-authored-by: David Allsopp <david.allsopp@metastack.com>

src/state/opamFileTools.ml

@@ -1192,10 +1192,25 @@ let read_opam dir =
       (OpamPp.string_of_bad_format (OpamPp.Bad_format (snd err)));
     None
   | None, None -> None
-  | exception (OpamPp.Bad_version _bf) ->
-    log "opam-version unsupported on %s. Add as dummy unavailable package."
+  | exception (OpamPp.Bad_version (_, version)) ->
+    TODO: here the version string is not a version but an error message
+    log "opam-version %S unsupported on %s. Added as dummy unavailable package." version
       (OpamFile.to_string opam_file);
-    Some (OpamFile.OPAM.with_available (FBool false) OpamFile.OPAM.empty)
+    if OpamVersion.compare OpamVersion.current (OpamVersion.of_string version) >= 0 then begin
+      log "Unexpected failure: failed to parse version %S but is below the current version %S"
+        version (OpamVersion.to_string OpamVersion.current);
+      Some (OpamFile.OPAM.with_available (FBool false) OpamFile.OPAM.empty)
+    end else
+      Some
+        (OpamFile.OPAM.with_descr_body
+           (Printf.sprintf
+              "This package uses opam 3.0 file format which opam 2.2 cannot read.\n\
+               \n\
+               In order to install or view information on this package, please upgrade your opam installation to at least version %s."
+              version)
+           (OpamFile.OPAM.with_available
+              (FOp (FIdent ([], OpamVariable.of_string "opam-version", None), `Geq, FString version))
+              OpamFile.OPAM.empty))
 
 let read_repo_opam ~repo_name ~repo_root dir =
   let open OpamStd.Option.Op in