Balloonpanel should handle non-existent styles.

[kkrzton]

Steps to reproduce

1. Go to ​https://www.webtrix.be/moonocolor/ (you can also use build with this or different skin downloaded from ​http://ckeditor.com/builder).
2. Switch to source mode and paste html:

<h1>Content Accessibility Matters</h1><p><img src="https://c.cksource.com/ac/2/images/accessibility-matters-01.png" style="float:right" /></p>

3. Switch back to WYSIWYG mode.
4. Click a11ychecker button to run Accessibility Checker.

Expected result

The Accessibility Checker dialog opens. It is styled and positioned properly.

Actual result

The Accessibility Checker dialog opens. It is not styled and positioned properly.

Other details (browser, OS, CKEditor version, installed plugins)

The cause of this issue is the fact that balloonpanel has only 3 supported skins (​https://github.com/ckeditor/ckeditor-dev/tree/master/plugins/balloonpanel/skins) and for any other skin, invalid CSS is loading (see attached screenshot) so there is no styling applied to ballonpanel (for its insides, which are handled by a11ychecker plugin, the default skin is loaded, which is moono-lisa since 4.6.0).

I think balloonpanel should use default styles if stylesheet for specific skin is not present.

[kkrzton]

The only thing which changed in 4.6.0 is that the balloonpanel was moved as a core plugin. I assume this case might be a problem also in earlier versions.