generate_packets.py: assertions should be errors
Part of #43927. Current generate_packets.py contains a lot of assert statements, which only get run when __debug__ is true – most of these check the integrity of outside input (packets.def) and should thus raise ValueErrors instead.
alienvalkyrie 
Part of #43927. Current generate_packets.py contains a lot of assert statements, which only get run when __debug__ is true – most of these check the integrity of outside input (packets.def) and should thus raise ValueErrors instead.