Only add assertion messages on failure.
Only add assertion messages on failure.

Adding assertion messages when missing was slowing down the code
because of the repeated throwing at catching of exceptions.  Now
only add a stack trace if the assertion will fail.

Issue #288

Change-Id: Ic4ec3180df0178f1e3e0b25534d9e5bb018707a4

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