Minor cleanup of AssetWatcher
Minor cleanup of AssetWatcher

This change fixes a bug where an NPE was thrown if an AssetWatcher didn't have a listener and encountered an error.


KoriSamui/PlayN,ruslansennov/playn,Just-/playn,threerings/playn,threerings/playn,KoriSamui/PlayN,KoriSamui/PlayN,Just-/playn,KoriSamui/PlayN,Just-/playn,ruslansennov/playn,Just-/playn,threerings/playn,threerings/playn,Just-/playn,ruslansennov/playn