Changed collision warning for Traits to fatal error.

Changed collision warning for Traits to fatal error.
#This change is made to have all possible cases of collisions consistently handled as fatal errors.
#The reason to have it fatal is that most likely something changed unexpectedly and needs urgent attention by the developer, since it will fail eventually anyway for instance because the expected method is missing in the class.
#Discussed in this thread: http://marc.info/?l=php-internals&m=129155790226876