Return the error string associated with an mqtt error number.

def error_string(mqtt_errno):
    """Return the error string associated with an mqtt error number."""
    if mqtt_errno == MQTT_ERR_SUCCESS:
        return "No error."
    elif mqtt_errno == MQTT_ERR_NOMEM:
        return "Out of memory."
    elif mqtt_errno == MQTT_ERR_PROTOCOL:
        return "A network protocol error occurred when communicating with the broker."
    elif mqtt_errno == MQTT_ERR_INVAL:
        return "Invalid function arguments provided."
    elif mqtt_errno == MQTT_ERR_NO_CONN:
        return "The client is not currently connected."
    elif mqtt_errno == MQTT_ERR_CONN_REFUSED:
        return "The connection was refused."
    elif mqtt_errno == MQTT_ERR_NOT_FOUND:
        return "Message not found (internal error)."
    elif mqtt_errno == MQTT_ERR_CONN_LOST:
        return "The connection was lost."
    elif mqtt_errno == MQTT_ERR_TLS:
        return "A TLS error occurred."
    elif mqtt_errno == MQTT_ERR_PAYLOAD_SIZE:
        return "Payload too large."
    elif mqtt_errno == MQTT_ERR_NOT_SUPPORTED:
        return "This feature is not supported."
    elif mqtt_errno == MQTT_ERR_AUTH:
        return "Authorisation failed."
    elif mqtt_errno == MQTT_ERR_ACL_DENIED:
        return "Access denied by ACL."
    elif mqtt_errno == MQTT_ERR_UNKNOWN:
        return "Unknown error."
    elif mqtt_errno == MQTT_ERR_ERRNO:
        return "Error defined by errno."
    else:
        return "Unknown error."