unhandled exceptions should render better error messagea

[avsm]

It would be more informative if the compiler output Unhandled_effect <EFFECT_NAME> instead of Unhandled. Also, bonus points for a backtrace of where the perform happened, as happens with exceptions when CAMLRUNPARAM=b.

[kayceesrk]

Also, bonus points for a backtrace of where the perform happened, as happens with exceptions when CAMLRUNPARAM=b.

This is already done as unhandled effects raise Unhandled exception at the point of perform: https://github.com/ocaml-multicore/ocaml-multicore/blob/5.00/stdlib/effectHandlers.mli#L18-L21.

(* foo.ml *)
open EffectHandlers

type _ eff += E : unit eff

let rec foo () =
  perform E;
  foo ()
  [@@inline never]

let _ = foo ()

$ OCAMLRUNPARAM=b ./a.out 
Fatal error: exception Unhandled
Raised by primitive operation at Foo.foo in file "foo.ml", line 7, characters 2-11
Called from Foo in file "foo.ml", line 11, characters 8-14