Catch exceptions when getting the RequireJS src

Ref issue #72.

`$this->engine->supports` throws `InvalidArgumentException` when using PHP templates instead of Twig templates.

```
An exception has been thrown during the rendering of a template ("Template name "/cdnjs.cloudflare.com/ajax/libs/require.js/2.1.8/require.min.js" is not valid (format is "bundle:section:template.format.engine").") in HearsayRequireJSBundle::initialize.html.twig at line 46.
```