New error code for codec/container support issues

[joeyparrish]

Currently, if a browser doesn't support the container or codecs in a piece of content, the error code is UNPLAYABLE_PERIOD. While this is accurate, it is not clear enough to the developer.

The UNPLAYABLE_PERIOD error code should be used when some periods are playable and others are not. A new error code should be used when all periods (including single-period content) are unplayable. I suggest CONTENT_NOT_SUPPORTED_BY_BROWSER. This would make it clearer why the content can't be played.