Clarify the error message when we disconnect a peer

Clarify the error message when we disconnect a peer

We very regularly receive confusion over the super generic
"Peer sent invalid data or we decided to disconnect due to a
protocol error" message, which doesn't say very much. Usually, we
end up disconnecting because we have a duplicate connection with a
peer, which doesn't merit such a scary message.

Instead, here we clarify the error message to just refer to the
fact that we're disconnecting, and note that its usually a dup
connection in a parenthetical.