wallet: clarify error 302.

"Dust limit unmet" seems undescriptive to me.

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

common/wallet_tx.c

@@ -20,8 +20,9 @@ static bool check_amount(const struct wallet_tx *tx)
 		return false;
 	}
 	if (tx->amount < 546) {
-		command_fail(tx->cmd, FUND_DUST_LIMIT_UNMET,
-			     "Dust limit unmet");
+		command_fail(tx->cmd, FUND_OUTPUT_IS_DUST,
+			     "Output %"PRIu64" satoshis would be dust",
+			     tx->amount);
 		return false;
 	}
 	return true;

doc/lightning-fundchannel.7

@@ -2,12 +2,12 @@
 .\"     Title: lightning-fundchannel
 .\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
 .\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
-.\"      Date: 06/15/2018
+.\"      Date: 06/17/2018
 .\"    Manual: \ \&
 .\"    Source: \ \&
 .\"  Language: English
 .\"
-.TH "LIGHTNING\-FUNDCHANN" "7" "06/15/2018" "\ \&" "\ \&"
+.TH "LIGHTNING\-FUNDCHANN" "7" "06/17/2018" "\ \&" "\ \&"
 .\" -----------------------------------------------------------------
 .\" * Define some portability stuff
 .\" -----------------------------------------------------------------
@@ -86,7 +86,7 @@ The following error codes may occur:
 .sp -1
 .IP \(bu 2.3
 .\}
-302\&. The dust limit is not met\&.
+302\&. The output amount is too small, and would be considered dust\&.
 .RE
 .sp
 Failure may also occur if \fBlightningd\fR and the peer cannot agree on channel parameters (funding limits, channel reserves, fees, etc\&.)\&.

doc/lightning-fundchannel.7.txt

@@ -39,7 +39,7 @@ The following error codes may occur:
 * 300. The maximum allowed funding amount is exceeded.
 * 301. There are not enough funds in the internal wallet (including fees) to
        create the transaction.
-* 302. The dust limit is not met.
+* 302. The output amount is too small, and would be considered dust.
 
 Failure may also occur if *lightningd* and the peer cannot agree on channel
 parameters (funding limits, channel reserves, fees, etc.).

lightningd/jsonrpc_errors.h

@@ -32,7 +32,7 @@
 /* `fundchannel` or `withdraw` errors */
 #define FUND_MAX_EXCEEDED               300
 #define FUND_CANNOT_AFFORD              301
-#define FUND_DUST_LIMIT_UNMET           302
+#define FUND_OUTPUT_IS_DUST             302
 
 /* Errors from `invoice` command */
 #define INVOICE_LABEL_ALREADY_EXISTS	900