Better error handling for billboard if image cannot be loaded

Currently, if a billboard can't load an image (like if the URL returns a 404 error) it prints an error to the console. The application has no way of knowing if and when the image was loaded. It would be better if there was a promise or something that was publicly accessible.

This has come up a few times in the forum, most recently here: https://groups.google.com/forum/#!topic/cesium-dev/A1CCO5u53W4

Start from the billboard image-loading path and reproduce the 404 case described in the issue and forum thread. Determine how a publicly accessible completion or failure signal should cover both successful and failed image loads; done means callers can reliably observe when loading finishes or fails instead of relying on console output.

Written by the indexing model from the issue text.