Every few hours, relays [warn] Received circuit padding stop command for unknown machine.

toralf says on #tor-relays:

I got 5x within last 36 ours with 0.4.1.1-alpha:  [warn] Received circuit padding stop command for unknown machine.        -shall I ignore that?

I'm marking this as 041-must, because we will get lots of bug reports from relay operators if we release a stable with this warning. At the very least, we must downgrade it to a protocol error.