Send an error message back. Something like:
{"status": "error", "message": "invalid parameters"}
and let the API sort it out by itself, usually by running another call, etc.
You may also add some more info in the message about the nature of the error in order for the API know what to do, like:
{"status": "error", "message": "email is invalid. please ask user to provide valid email."}
Often times the API will be able to sort it out.