Add plugin placeholder when ppapi can’t be loaded.

Blink shows an error message in place of the plugin if error occures. With a plugin placeholder, it would look a little bit nicer and it may show more elaborate information.

For example, if the plugin is not supported:

./simplebrowser --register-pepper-plugins="/path/to/libppapi_example_paint_manager.so;application/x-ppapi-example-paint_manager" file:///path/to/ppapi/examples/2d/2d.html

 Actual:

Expected: