Use multiple BaseURLs for failover

[joeyparrish]

From the spec:

5.6.5 - Alternative base URLs

If alternative base URLs are provided through the BaseURL element at any level, identical Segments shall be accessible at multiple locations. In the absence of other criteria, the DASH Client may use the first BaseURL element as “base URI". The DASH Client may use base URLs provided in the BaseURL element as “base URI” and may implement any suitable algorithm to determine which URLs it uses for requests.

A.7 - Reaction to error codes

If multiple BaseURL elements are available, the client may also check for alternative instances of the
same content that are hosted on a different server.