I don't advice you to do this, it's vert dangerous and there are lot of chances you brick your 3ds, so wait the next GW update
Someone in a French forum had the same problem as you, he tried to turn off his 3DS just before this error appears, then he turned it on, he launched the exploit and he launched the downgrade process then it worked.
Someone in a French forum had the same problem as you, he tried to turn off his 3DS just before this error appears, then he turned it on, he launched the exploit and he launched the downgrade process then it worked.