Element.ShadowRoot returns error when ShadowRoots is empty (#788)

* Element.ShadowRoot returns an error when ShadowRoots slice is empty

* comment exported ErrNoShadowRoot

* unit test shadow root error

* test shadow root error contains correct error message

element.go

@@ -402,6 +402,9 @@ func (el *Element) ShadowRoot() (*Element, error) {
 	}
 
 	// though now it's an array, w3c changed the spec of it to be a single.
+	if len(node.ShadowRoots) == 0 {
+		return nil, &ErrNoShadowRoot{el}
+	}
 	id := node.ShadowRoots[0].BackendNodeID
 
 	shadowNode, err := proto.DOMResolveNode{BackendNodeID: id}.Call(el)

element_test.go

@@ -284,6 +284,11 @@ func TestShadowDOM(t *testing.T) {
 		g.mc.stubErr(1, proto.DOMResolveNode{})
 		el.MustShadowRoot()
 	})
+
+	elNoShadow := p.MustElement("script")
+	_, err := elNoShadow.ShadowRoot()
+	g.True((&rod.ErrNoShadowRoot{}).Is(err))
+	g.Has(err.Error(), "element has no shadow root:")
 }
 
 func TestInputTime(t *testing.T) {

error.go

@@ -181,3 +181,16 @@ type ErrPageNotFound struct {
 func (e *ErrPageNotFound) Error() string {
 	return "cannot find page"
 }
+
+// ErrNoShadowRoot error
+type ErrNoShadowRoot struct {
+	*Element
+}
+
+// Error ...
+func (e *ErrNoShadowRoot) Error() string {
+	return fmt.Sprintf("element has no shadow root: %s", e.String())
+}
+
+// Is interface
+func (e *ErrNoShadowRoot) Is(err error) bool { _, ok := err.(*ErrNoShadowRoot); return ok }