Assets.js: Now with exception handling!

Changes:
-- All internal logging now uses the updated jaws.log functionality
(calls to jaws.log.warn and jaws.log.error)

-- Each resource loading section {Image, Audio, Video} has its own
try-catch block and error reporting.

-- assetLoaded (internal to jaws.assets) now has its own try-catch for
catching partial downloads or malformed responses from XHR requests.