Fix handling of internal-error key status

[joeyparrish]

We have been operating under the assumption that the internal-error key status should not happen unless something catastrophic happens in the CDM. This is not true.

We should treat the keys marked internal-error the same as output-restricted: ignore tracks that require those keys. So long as there are usable keys, we should be able to play.

shaka.util.Error.Code.INTERNAL_ERROR_KEY_STATUS should be removed.