channeldb: return error when payment is not initialized

This commit changes `fetchPaymentStatus` to return an error when the
payment creation info bucket cannot be found. Instead of using the
ambiguous status `StatusUnknown`, we tell the callsite explictly that
there's no such payment. This also means the vague `StatusUnknown` can
now be removed as it has multiple meanings.

lnrpc/routerrpc/router_backend.go

@@ -1370,9 +1370,6 @@ func convertPaymentStatus(dbStatus channeldb.PaymentStatus) (
 	lnrpc.Payment_PaymentStatus, error) {
 
 	switch dbStatus {
-	case channeldb.StatusUnknown:
-		return lnrpc.Payment_UNKNOWN, nil
-
 	case channeldb.StatusInFlight:
 		return lnrpc.Payment_IN_FLIGHT, nil