Errors inside a sandbox can be propagated to the container.

Inside a sandbox, use postMessage to send an event to the container.
The error is contained in the `sandboxException` attribute of the
message.

Example for an iframe:
```js
  Window.postMessage( window.parent, {sandboxException: "oups"}, "*" );
```

Override the `onerror` handler on the sandbox to handle the error.

```js
  sandbox.onerror = function( error ) {
    // Do what you need to do
  };
```
Update MessageChannel.js to 0.1.4
JS iframes can't load resources from an origin  different than the one from the bootstrap
Merge pull request #71 from tildeio/check_resources

JS iframes can't load resources from a domain different than the one from the bootstrap