Improve env error message

src/const.ml

@@ -48,7 +48,7 @@ let get_var: ?default:(string lazy_t) -> string lazy_t -> string lazy_t =
     let open Lazy in
     let msg =
       name >>| fun name ->
-      sprintf "ERROR: Couldn't get %s. Please consider setting it." name
+      sprintf "ERROR: Could not get $%s. Please consider setting it." name
     in
     (* Get the var *)
     name >>= fun name ->