The same catch block is there in wasm_exec.js for node processes.
Added it in browser invocations too, to prevent uncaught exceptions.

Change-Id: Icab577ec585fa86df3c76db508b49401bcdb52ae
Reviewed-on: https://go-review.googlesource.com/132916Reviewed-by: Richard Musiol <neelance@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>