Handle JSON parse errors for parsing data-config attribute

Please review this before accepting it, because I do not know of any implications of this change (it could just load the default configuration, or it could blow up).

The HTML for the CKEditor widget is:
<textarea class="django-ckeditor-textarea" cols="40" data-config-name="default" id="id_content" name="content" rows="10"></textarea>

I'm not sure if I should include a data-config attribute on the textarea tag or not, but this got the widget loading for me.