Fix Studio to gracefully handle xblock JavaScript errors

TNL-46

I've changed Studio to catch JavaScript errors when rendering xblocks, log the error, but to then continue as normal. This means that the user is still able to interact with the xblock to delete, duplicate etc. This seems reasonable as the xblock is only rendered as a WYSIWYG representation so if it isn't fully interactive that shouldn't be a big problem.
In `edit_xblock.js` you declare runtime like so: `runtime = xblock ? xblock.runtime : null`. I'm not sure which style I like better, but maybe you could use the same statement in both cases.