Download errors have no id

[LloydW93]

Previously, in error events, e.event.id had values such as "manifest", "initialization", "content", to identify the type of failure.

In 2.1.1, id is always "undefined", in the sense that it has been initialised to that value.

Sample steps to reproduce:

• Load http://dashif.org/reference/players/javascript/v2.1.1/samples/dash-if-reference-player/index.html?url=http://rdmedia.bbc.co.uk/dash/ondemand/testcard/1/client_manifest-events.mpd
• Break your network connection during playback
• Observe the value of e.event.id when the next segments fail to download

This appears to be a bug with XHRLoader.