Disconnecting a browser sends `browser_error` event to reporters

### Expected behaviour

Disconnecting a browser shouldn't send `browser_error` to reporters.

### Actual behaviour

Start karma server and capture a browser manually. Once a test run is done, close the browser manually.
As a result, reporter's `onBrowserError` method is called back with `error='Disconnectedundefined'`: https://github.com/karma-runner/karma/blob/v1.7.1/lib/browser.js#L40

Seems there is no much sense in this event after the test run. The problem is that reporters should filter out such events as it's not an error.

### Environment Details

- Karma version (output of `karma --version`): 1.7.1