1. A data processing apparatus for executing, in response to a processing execution instruction transmitted from an information terminal connected to the data processing apparatus via a network, processing in accordance with the processing execution instruction, the data processing apparatus comprising: a protocol determining unit configured to determine a protocol used in transmission of the processing execution instruction when an error is caused during execution of the processing; a converting unit configured to convert information on the error in accordance with the protocol determined by the protocol determining unit so that the information is compliant with the protocol; and a transmitting unit configured to transmit the information on the error converted by the converting unit to the information terminal that has transmitted the processing execution instruction using the protocol used in transmission of the processing execution instruction.