> I use a syntax extension that catches "Not_found" and raises a failure
> instead, with the source location of the "real" offending call. I do
> this mostly because OUnit catches exceptions so backtraces are of no
> use.

I have encoutered the same problem and resolved it with explicit
backtrace handling in Printexc. I use the following function wrapper :

let verbose_func func x =
    try func x with exn ->
      Printf.printf "Test error %s\n%!" (Pinrtexc.to_string exn);
      Printf.printf "%s\n%!" (Printexc.get_backtrace ());
      raise exn in