Condition Runtime.dump execution for JSOO-compiled binaries on the compiler backend used rather than exception raised

[pat-rondon]

Generalizes the fix of #334 for js_of_ocaml_integration:

• Adds a test demonstrating that, when running under js_of_ocaml and linking its runtime library, the expected Failure exception is not raised, and a Jsoo_runtime.Error is raised instead. This test is identical to the existing js_of_ocaml test except that it also links in the JSOO runtime library.
• Changes the logic for skipping the call to Runtime.dump to skip the call when the backed is js_of_ocaml, rather than when the Failure exception is raised.

[aantron]

Thank you!

I'll wait a bit for #396 and then do a release with this.

[pat-rondon]

Thanks!