Add message in case of error

Author: bobot

packages/conf-flint/conf-flint.3.0/opam

@@ -33,7 +33,8 @@ depexts: [
 ]
 synopsis: "Virtual package relying on a Flint lib system installation"
 description:
-  "This package can only install if the Flint lib is installed on the system."
+  "This package can only install if the Flint lib version 3 is installed on the system."
+post-messages: "You can try to install conf-flint.1 which works with version 2 of the library" { failure }
 authors: ["William Hart" "Sebastian Pancratz" "Andy Novocin" "Fredrik Johansson"
           "Tom Bachmann" "Mike Hansen" "Daniel Schultz"]
 flags: conf