Implemented error handling for failed xblocks

Previously, XBlocks that failed to load simply wouldn't appear on the workbench. Now, failed XBlocks, if present, have a traceback that can be seen on the workbench index. @pmitros @nedbat 

__Note__: This relies on XBlock pull request [240](https://github.com/edx/XBlock/pull/240), so this should be merged after that request gets merged onto master. Thus, the Travis CI build will fail until then.
Great point. I've fixed it to handle any exception correctly, not fundamentally relying on `args`. (Before, `raise Exception()` in an XBlock would cause a django-wide exception, now it is treated safely)