Lib: small refactoring around js exception printing

[hhugo]

No description provided.

[TyOverby]

Does this change any semantics?

[hhugo]

Previously, We would call toString to print any value that doesn't look like an OCaml exception.
With this PR, the printer only looks for js Errors.

So yes, there is a semantic difference. It was extracted from the wasm backend branch. Maybe @vouillon can tell if/why it is necessary for the wasm backend.

[vouillon]

If the value was an JavaScript array, we would consider it as an OCaml exception. Otherwise, we would use toString to turn get an error message, assuming that it was a JavaScript exception. Since does not work with wasm_of_ocaml, since OCaml exceptions are Wasm arrays, which are not JavaScript array and does not have a toString method. With this change, we instead test whether the value is a JavaScript exception.

I'm not sure we still need this code, actually. We now call caml_wrap_exception systematically to turn JavaScript exceptions into OCaml exceptions, so this code should never see any JavaScript exception.