Throw HTTP 400 error when idsite is invalid

#6661 When `idsite` is set to 0 or incorrectly, Piwik Tracker requests will send a HTTP 500 error. This causes monitoring system to be triggered when in fact this is not a server error 500 but rather it is a error `400 Bad Request`. Also if forced idVisitor is wrong. Not sure if there are many other use cases we'll have to add them on demand