Handle XML parsing error sensibly

Created by: davemevans

While testing another feature I discovered that XML parsing errors are handled differently across platforms and not caught in a sensible way on any. This could lead to the player attempting to continue with an incomplete manifest (Chrome, Safari) or fail but not for the correct reason (Firefox, but only because its error response contains a Processing Instruction node which causes the json conversion to barf).

This PR reliably detects parsererrors and forces a sensible (undefined) response from parser which makes the player act as I believe was intended. IE stop attempting to play the malformed manifest.

Please note I have NOT tested this on IE11/Edge as I don't have access to those browsers right now. Could someone try this out and let me know?