Handle XML parsing error sensibly

[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?

[KozhinM]

@bbcrddave, everything seems to work fine in Edge, but IE11 throws syntax error at the following code:
// src/lib/xml2json.js
ns = parser.parseFromString('<', 'text/xml')
Looks like IE just throws an exception when it is unable to parse xml.
I have not got a solution for this right now, but I will let you know if I come up with an idea.

[davemevans]

@KozhinM Thanks for looking! Surprised about IE11 as I was under the impression DOMParser was not supposed to throw any exceptions.

I've updated the solution with some extra logic for IE11. (Note that I tested it by isolating the functions and running it in Win7/IE11 rather than as part of dash.js)

[KozhinM]

@bbcrddave, thanks for a quick response. The fix works fine. I'll merge the PR right away.