Warn user if validation cannot be performed due to CORs headers

[thenewguy]

It isn't very intuitive when the validator fails with Error loading the MPD, please check the URL.

It would be helpful to add a CORs check so that the error can be acted upon.

It would probably also help to document this requirement. I wrongly assumed fetching would happen on the server in PHP, not the client in JS.

[dsilhavy]

That requires the client to know that the request failed because of CORS. Investigate if this is possible