lightningd: fix failure message in waitsendpay with multi-part payments.

Bastien TEINTURIER <bastien@acinq.fr> writes:
> One thing I noticed but didn't investigate much: after sending the two
> payments, I tried using `waitsendpay` and it reported an error *208*
> (*"Never attempted payment for
> '98ee736d29d860948e436546a88b0cc84f267de8818531b0fdbe6ce3d080f22a'"*).
> 
> I was expecting the result to be something like: "payment succeeded for
> that payment hash" (the HTLCs were correctly settled).

Indeed, if you waitsendpay without specifying a partid, you are waiting
for 0, which may not exist.  Clarify the error msg.

Reported-by: @t-bast
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/pay.c

@@ -627,7 +627,9 @@ static struct command_result *wait_payment(struct lightningd *ld,
 					 payment_hash, partid);
 	if (!payment) {
 		return command_fail(cmd, PAY_NO_SUCH_PAYMENT,
-				    "Never attempted payment for '%s'",
+				    "Never attempted payment part %"PRIu64
+				    " for '%s'",
+				    partid,
 				    type_to_string(tmpctx, struct sha256,
 						   payment_hash));
 	}