Always run with pcall, and only print stack trace if no user trace back is set

See a discussion of this here: http://getmoai.com/forums/in-moai-1-5-lua-errors-no-longer-display-the-lua-stack-t2497/

Basicly now Moai doesnt run with pcall if the user has not set a trace back. If the user has set a trace back, it does run with pcall, but no matter what the user traceback does it still prints its own stack trace as well. So you get a double stack trace.

I cant see a situation where you would not want a stack trace so I made it always use pcall, and only print stack trace when there is not user traceback set.