Runtime: abort instead of exit on unimplemented js primitives

[hhugo]

fix #677

[dbuenzli]

I think it would be better to use caml_fatal_error and eschew the fprintf. This would hook into the support provided by the runtime system (notably you can easily define a caml_fatal_error_hook to do what you want with the error reporting). See the discussions here.

[hhugo]

Thanks, changed in https://github.com/ocsigen/js_of_ocaml/commit/28743eeb8a6428e9d82f4051512b9fb708e5af29

[dbuenzli]

Thank you!