@NonNull
	public Request invalid(@NonNull final InvalidRequestCallback callback) {
		this.invalidRequestCallback = callback;
		return this;
	}
Use to set a callback that will be called in case the request was invalid, for example
called before the device was connected.
This callback will be ignored if request was executed synchronously, in which case
the error will be returned as an exception.

@param callback the callback.
@return The request.
ble/src/main/java/no/nordicsemi/android/ble/Request.java