get rid of all exceptions

We hadn't noticed a few uncaught exceptions being received from the
browser, because the events were ignored. Start printing them via the
error logger.

The ones we were getting were caused by testAllocate running Navigate
actions when the path argument was empty. Navigating to "testdata/"
causes JS exceptions, as it's not a valid page.

Instead, leave the new target pointing at a blank document.

browser.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/chromedp/cdproto"
 	"github.com/chromedp/cdproto/cdp"
+	"github.com/chromedp/cdproto/runtime"
 	"github.com/chromedp/cdproto/target"
 	"github.com/mailru/easyjson"
 )
@@ -203,6 +204,14 @@ func (b *Browser) run(ctx context.Context) {
 					// these events?
 					continue
 				}
+				if msg.Method == cdproto.EventRuntimeExceptionThrown {
+					ev := new(runtime.EventExceptionThrown)
+					if err := json.Unmarshal(msg.Params, ev); err != nil {
+						b.errf("%s", err)
+						continue
+					}
+					b.errf("%+v\n", ev.ExceptionDetails)
+				}
 
 				page, ok := b.pages[sessionID]
 				if !ok {

chromedp_test.go

@@ -23,8 +23,11 @@ var (
 func testAllocate(t *testing.T, path string) (_ context.Context, cancel func()) {
 	ctx, cancel := NewContext(allocCtx)
 
-	if err := Run(ctx, Navigate(testdataDir+"/"+path)); err != nil {
-		t.Fatal(err)
+	// Only navigate if we want a path, otherwise leave the blank page.
+	if path != "" {
+		if err := Run(ctx, Navigate(testdataDir+"/"+path)); err != nil {
+			t.Fatal(err)
+		}
 	}
 
 	//if err := WithLogf(t.Logf)(c.c); err != nil {

nav_test.go

@@ -56,14 +56,14 @@ func TestNavigationEntries(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if len(entries) != 2 {
-		t.Errorf("expected to have 2 navigation entry: got %d", len(entries))
+	if len(entries) != 1 {
+		t.Errorf("expected to have 1 navigation entry: got %d", len(entries))
 	}
-	if index != 1 {
-		t.Errorf("expected navigation index is 1, got: %d", index)
+	if index != 0 {
+		t.Errorf("expected navigation index is 0, got: %d", index)
 	}
 
-	expIdx, expEntries := 2, 3
+	expIdx, expEntries := 1, 2
 	for i, url := range tests {
 		if err := Run(ctx, Navigate(testdataDir+"/"+url)); err != nil {
 			t.Fatal(err)
@@ -77,7 +77,7 @@ func TestNavigationEntries(t *testing.T) {
 		if len(entries) != expEntries {
 			t.Errorf("test %d expected to have %d navigation entry: got %d", i, expEntries, len(entries))
 		}
-		if want := int64(i + 2); index != want {
+		if want := int64(i + 1); index != want {
 			t.Errorf("test %d expected navigation index is %d, got: %d", i, want, index)
 		}