give better errors on unexpected undefined objects

First, if Evaluate finds one, give a helpful error instead of json's
unhelpful one.

Second, make JavascriptAttribute give a bit of context as part of its
error, so that the user can see what attribute caused the Evaluate
error.

Fixes #410.

eval.go

@@ -3,6 +3,7 @@ package chromedp
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 
 	"github.com/chromedp/cdproto/runtime"
 )
@@ -63,6 +64,13 @@ func Evaluate(expression string, res interface{}, opts ...EvaluateOption) Evalua
 			return nil
 		}
 
+		if v.Type == "undefined" {
+			// The unmarshal above would fail with the cryptic
+			// "unexpected end of JSON input" error, so try to give
+			// a better one here.
+			return fmt.Errorf("encountered an undefined value")
+		}
+
 		// unmarshal
 		return json.Unmarshal(v.Value, res)
 	})

query.go

@@ -877,7 +877,12 @@ func JavascriptAttribute(sel interface{}, name string, res interface{}, opts ...
 			return fmt.Errorf("selector %q did not return any nodes", sel)
 		}
 
-		return EvaluateAsDevTools(snippet(attributeJS, cashX(true), sel, nodes[0], name), res).Do(ctx)
+		if err := EvaluateAsDevTools(
+			snippet(attributeJS, cashX(true), sel, nodes[0], name), res,
+		).Do(ctx); err != nil {
+			return fmt.Errorf("could not retrieve attribute %q: %v", name, err)
+		}
+		return nil
 	}, opts...)
 }
 

query_test.go

@@ -502,6 +502,21 @@ func TestValue(t *testing.T) {
 	}
 }
 
+func TestValueUndefined(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := testAllocate(t, "form.html")
+	defer cancel()
+
+	var value string
+	err := Run(ctx, Value("foo", &value, ByID))
+	want := `could not retrieve attribute "value": encountered an undefined value`
+	got := fmt.Sprint(err)
+	if !strings.Contains(got, want) {
+		t.Fatalf("want error %q, got %q", want, got)
+	}
+}
+
 func TestSetValue(t *testing.T) {
 	t.Parallel()