Add the ThrowException fn to the isolate

[wisepythagoras]

I've re-implemented the ThrowException function from #143, but added support for passing in a Value instead of just a string, as pointed out by @dylanahsmith. This allows us to pass a string, or forward in a TypeError.

A note: The return value is always null or undefined.

[codecov]

Codecov Report

Merging #221 (8252b3e) into master (4a6bcfb) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   95.83%   95.86%   +0.02%     
==========================================
  Files          17       17              
  Lines         576      580       +4     
==========================================
+ Hits          552      556       +4     
  Misses         15       15              
  Partials        9        9              

Impacted Files	Coverage Δ
isolate.go	100.00% <100.00%> (ø)

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[genevieve]

Might be able to test it by calling the function from within another function like:

func TestIsolateThrowException(t *testing.T) {
	t.Parallel()
	iso := v8.NewIsolate()
	defer iso.Dispose()

	typeErr, err := v8.NewValue(iso, "some type error")
	if err != nil {
		t.Errorf("unexpected error: %v", err)
	}

	fn := v8.NewFunctionTemplate(iso, func(info *v8.FunctionCallbackInfo) *v8.Value {
		v := iso.ThrowException(typeErr)
		if !v.IsNull() && !v.IsUndefined() {
			t.Error("expected result to be <nil> or undefined")
		}
		return nil
	})

	global := v8.NewObjectTemplate(iso)
	global.Set("foo", fn)

	ctx := v8.NewContext(iso, global)
	defer ctx.Close()

	_, e := ctx.RunScript("foo()", "forever.js")
	if e.Error() != "some type error" {
		t.Errorf("expected error but got: %v", e)
	}
}

[wisepythagoras]

Thanks, @genevieve. I had forgotten about the error returned by RunScript. I modified it a little to include a test case for a TypeError.