stun, stunner: clarify an error log message more

But two earlier changes mean this doesn't show up anymore anyway.
But if it does, it'll be a nice message.

stun/stun.go

@@ -124,7 +124,7 @@ func ParseBindingRequest(b []byte) (TxID, error) {
 
 var (
 	ErrNotSTUN            = errors.New("response is not a STUN packet")
-	ErrNotSuccessResponse = errors.New("STUN response error")
+	ErrNotSuccessResponse = errors.New("STUN packet is not a response")
 	ErrMalformedAttrs     = errors.New("STUN response has malformed attributes")
 	ErrNotBindingRequest  = errors.New("STUN request not a binding request")
 	ErrWrongSoftware      = errors.New("STUN request came from non-Tailscale software")

stunner/stunner.go

@@ -110,7 +110,7 @@ func (s *Stunner) Receive(p []byte, fromAddr *net.UDPAddr) {
 			// check probe coming in late. Ignore.
 			return
 		}
-		s.logf("stunner: received bad STUN response: %v", err)
+		s.logf("stunner: received unexpected STUN message response from %v: %v", fromAddr, err)
 		return
 	}
 	r, ok := s.removeTX(tx)