Pigeon Series 999 1.6.0

• Added new Exhaust Tuning option to Sound Modification slot (uses the new race I4 exhaust sound)
• Reworked Sound Modification slot to overwrite bike I4s' predetermined engine sounds instead of adding them, so the Empty option is now actually empty
• Updated Sound Modification slot for all configs (now some are equipped with Exhaust Tuning)
• Fixed errors thrown by console for non-existent taillight and heat shield base materials (inconsequential but still mildly annoying)