@NonNull
	public Request fail(@NonNull final FailCallback callback) {
		this.failCallback = callback;
		return this;
	}
Use to set a callback that will be called in case the request has failed.
If the target device wasn't set before executing this request
({@link BleManager#connect(BluetoothDevice)} was never called), the
{@link #invalid(InvalidRequestCallback)} will be used instead, as the
{@link BluetoothDevice} is not known.
<p>
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