RQS-25185 nil handling

@krobi64 @shunchu 

couldn't escape nil, but I'm not sure if I'm just kicking the nil down the road but this makes a proper error bubble up:

BlueKaiClient::InvalidServiceRequest - {:errors=>{:audience_id=>["AUDIENCE_ID_INVALID"]}}:

If this okay, I can rescue in Buyer audiences#price. Should I pass the error on as json or just give a 400 error?