go_js_wasm_exec was returning with code 0 if WebAssembly.instantiate
failed. This made failing tests show as passed.

Change-Id: Icfb2f42e9f1c3c70ca4a130a61a63cb305edff32
Reviewed-on: https://go-review.googlesource.com/c/go/+/168885


Run-TryBot: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>