* 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
