Add assertion messages to StreamGenerator
Add assertion messages to StreamGenerator

Change-Id: I2af2eb8e4e0a6b92aca1611ba1b8b4af78c90eeb

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