[PERF-303] Integer XBlocks/XModules into the static asset pipeline.
This PR, based on hackathon work from Christina/Andy, implements a way to discover all installed XBlocks and XModules and to enumerate their public assets, then pulling them in during the collectstatic phase and hashing them. In turn, the methods for generating URLs to resources will then returned the hashed name for assets, allowing them to be served from nginx/CDNs, and cached heavily.
* call error tracker when needed
* remove duplicate logging--just add info and re-raise
* xml modulestore uses error tracker to capture load errors
* add unstyled list of import errors to courseware homepage!
* simplify logic--tracker just tracks errors. Trackers should not raise,
and are not be responsible for logging.
* adapted code to use trackers.
* Started cleanup of error handling code:
- if need to add info and re-raise, just do that. No logging.
- if working around a problem, log and track as needed.
* add error_handler member to DescriptorSystem
* call it where import errors happen
* also includes some refactoring in xml.py
* some more line length and docstring cleanups
