Dispatch network error when HEAD request fails
Dispatch network error when HEAD request fails

UNABLE_TO_GUESS_MANIFEST_TYPE doesn't indicate that the guesswork
failed due to a network error.  To make it clearer what went wrong,
dispatch the original network error from the HEAD request.

Change-Id: I1905afdf0913740f9fce0d011b6b2ac3ad37efd7

TheModMaker/shaka-player,TheModMaker/shaka-player,brightcove/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,TobbeEdgeware/shaka-player,TheModMaker/shaka-player,indiereign/shaka-player,vimond/shaka-player,Ross-cz/shaka-player,russitto/shaka-player,shaka-project/shaka-player,russitto/shaka-player,russitto/shaka-player,brightcove/shaka-player,TobbeEdgeware/shaka-player,TheModMaker/shaka-player,shaka-project/shaka-player,brightcove/shaka-player,vimond/shaka-player,TobbeEdgeware/shaka-player,shaka-project/shaka-player,indiereign/shaka-player,Ross-cz/shaka-player,Ross-cz/shaka-player,shaka-project/shaka-player,indiereign/shaka-player,vimond/shaka-player