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.