peer: display lnwire.Error body as ascii string in messageSummary

It dictates in the spec, that the error message should be an ASCII
string to allow other implementations to easily discern the type of
error. The other implementations do this, but we don’t yet, but we’ll
go ahead and display it anyway as it’s helpful when debugging.

peer.go

@@ -864,7 +864,7 @@ func messageSummary(msg lnwire.Message) string {
 			msg.ChanID, msg.ID, msg.FailureCode)
 
 	case *lnwire.Error:
-		return fmt.Sprintf("chan_id=%v, err=%v", msg.ChanID, msg.Data)
+		return fmt.Sprintf("chan_id=%v, err=%v", msg.ChanID, string(msg.Data))
 
 	case *lnwire.AnnounceSignatures:
 		return fmt.Sprintf("chan_id=%v, short_chan_id=%v", msg.ChannelID,