Remove use of Printexc.catch from debugger

Allow the runtime to display details of any uncaught exception (with
backtraces, if enabled). Unix.handle_unix_error is still used to convert
errors from system calls to a less unmeaningful form.

debugger/main.ml

@@ -245,4 +245,4 @@ let main () =
       exit 2
 
 let _ =
-  Printexc.catch (Unix.handle_unix_error main) ()
+  Unix.handle_unix_error main ()