respond with 200/success when participant/network-event uniqueness validation fails
respond with 200/success when participant/network-event uniqueness validation
fails

Any other error should be handled the same way it was (ie a 400 with
appropriate error message)

jzw/network,tmiller/network,TadBirmingham/network,andreburto/network,jzw/network,jzw/network,TadBirmingham/network,andreburto/network,andreburto/network,TadBirmingham/network,tmiller/network,tmiller/network