dusty htlcs: don't fail the channel, make it error a whole bunch

Let's make this a softer launch by just warning on the channel til the
feerates go back down.

You can also 'fix' this by upping your dust limit with
the `max-dust-htlc-exposure-msat` config.

channeld/channeld.c

@@ -1310,10 +1310,10 @@ static void send_commit(struct peer *peer)
 			/* Is this feerate update going to push the committed
 			 * htlcs over our allowed dust limits? */
 			if (!htlc_dust_ok(peer->channel, feerate_target, REMOTE)
-			   || !htlc_dust_ok(peer->channel, feerate_target, LOCAL))
-				/* We fail the channel. Oops */
-				peer_failed_err(peer->pps, &peer->channel_id,
-						"Too much dust to update fee");
+			    || !htlc_dust_ok(peer->channel, feerate_target, LOCAL))
+				peer_failed_warn(peer->pps, &peer->channel_id,
+						"Too much dust to update fee (Desired"
+						" feerate update %d)", feerate_target);
 
 			if (!channel_update_feerate(peer->channel, feerate_target))
 				status_failed(STATUS_FAIL_INTERNAL_ERROR,

tests/test_pay.py

@@ -2493,9 +2493,9 @@ def test_htlc_too_dusty_outgoing(node_factory, bitcoind):
     l1.set_feerates([feerate * 2] * 4, False)
     l1.restart()
 
-    # the channel should fail -- too much dust
+    # the channel should start warning -- too much dust
     inv = l2.rpc.invoice(htlc_val_msat, str(num_dusty_htlcs + 1), str(num_dusty_htlcs + 1))
-    with pytest.raises(RpcError, match=r'WIRE_UNKNOWN_NEXT_PEER'):
+    with pytest.raises(RpcError, match=r'WIRE_TEMPORARY_CHANNEL_FAILURE'):
         l1.rpc.sendpay(route, inv['payment_hash'], payment_secret=inv['payment_secret'])