misc/wasm: handle error during instantiateStreaming

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>

misc/wasm/wasm_exec.html

@@ -27,6 +27,8 @@ license that can be found in the LICENSE file.
 			mod = result.module;
 			inst = result.instance;
 			document.getElementById("runButton").disabled = false;
+		}).catch((err) => {
+			console.error(err);
 		});
 
 		async function run() {