Load bootstrape.lua in conf.lua.

[MOD] Load bootstrape.lua in conf.lua
[ENH] Use local reference in bootstrape.lua.
[NEW] Game.exit() is actually exit the game and Game.quit() will promot
a confirm dialog.
[NEW] Now use love.errhand and love.releaseerrhand to handle errors.
[ENH] Improve the code in main.lua.
[ENH] Improve the code in Game.start().
[DEL] Remove SceneError.