renderingqueue 'not ready' exception blocking & masking lua scripts errors

[NevilClavain]

when a lua script encount an error between node registration in rendering queue and a rendering queue update() call, this one raise an exception, stopping gameroom and preventing lua error text display. We shall change this behaviour by :

• removing exception throw from rendering queue and replacing it by a error stored in a rendering queue class string member; in the same time rendering queue continues to loop but doing nothing, rather than throwing an execption
• adding accessor to lua class to check rendering queue error status from lua