cmd/trace: don't fail when no browser is available

When there is no browser available on the system,
we should print the URL instead of failing.

Change-Id: I4a2b099e17609394273eff150062c285d76bbac1
Reviewed-on: https://go-review.googlesource.com/13774Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/cmd/trace/main.go

@@ -71,7 +71,7 @@ func main() {
 	}
 	// Open browser.
 	if !startBrowser("http://" + ln.Addr().String()) {
-		dief("failed to start browser\n")
+		fmt.Fprintf(os.Stderr, "Trace viewer is listening on http://%s\n", ln.Addr().String())
 	}
 
 	// Parse and symbolize trace asynchronously while browser opens.