Hide OnLoadError message from end users

The shell currently displays URL loading errors to end users, it's not pretty. For example, using dev tools  `window.location.href = "unsupported://"` you'll see:

![image](https://cloud.githubusercontent.com/assets/1148713/4342843/f8025526-404a-11e4-8457-6852f60bf626.png)

This PR does 2 things: (1) use a background color that matches the title bar (and working set bgcolor) and (2) suppress the error in an HTML comment `<!-- -->` for developers to find in dev tools:

![image](https://cloud.githubusercontent.com/assets/1148713/4342858/1b6d9962-404b-11e4-815c-2ab99bb425a0.png)

Note: Pretend there's comment markup in that last screenshot.