Better error message when ocamlformat emit source triggering warning 50

Author: hhugo

lib/Translation_unit.ml

@@ -62,7 +62,8 @@ module Error = struct
   let print_internal_error ~debug ~quiet fmt e =
     let s =
       match e with
-      | `Cannot_parse _ -> "generating invalid ocaml syntax"
+      | `Cannot_parse (Parse_with_comments.Warning50 _) -> "generating invalid comment attachment"
+      | `Cannot_parse e -> "generating invalid ocaml syntax"
       | `Ast_changed -> "ast changed"
       | `Comment _ -> "comment changed"
       | `Warning50 _ -> "misplaced documentation comment"