funding: don't allow funding new channels until we're synced.

This is probably worth preventing.

1. Our depth estimate would be inaccurate possibly leading to us
   timing out too early.
2. If we're not up-to-date our onchain funds are unknown.
3. We wouldn't be able to send or receive HTLCs until we're synced anyway.

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

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: 05/24/2019
+.\"      Date: 08/07/2019
 .\"    Manual: \ \&
 .\"    Source: \ \&
 .\"  Language: English
 .\"
-.TH "LIGHTNING\-FUNDCHANN" "7" "06/07/2019" "\ \&" "\ \&"
+.TH "LIGHTNING\-FUNDCHANN" "7" "08/07/2019" "\ \&" "\ \&"
 .\" -----------------------------------------------------------------
 .\" * Define some portability stuff
 .\" -----------------------------------------------------------------
@@ -110,6 +110,17 @@ The following error codes may occur:
 303\&. Broadcasting of the funding transaction failed, the internal call to bitcoin\-cli returned with an error\&.
 .RE
 .sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+304\&. We\(cqre still syncing with the bitcoin network\&. Retry later\&.
+.RE
+.sp
 Failure may also occur if \fBlightningd\fR and the peer cannot agree on channel parameters (funding limits, channel reserves, fees, etc\&.)\&.
 .SH "SEE ALSO"
 .sp

doc/lightning-fundchannel.7.txt

@@ -62,6 +62,7 @@ The following error codes may occur:
 * 302. The output amount is too small, and would be considered dust.
 * 303. Broadcasting of the funding transaction failed, the internal call to
        bitcoin-cli returned with an error.
+* 304. We're still syncing with the bitcoin network.  Retry later.
 
 Failure may also occur if *lightningd* and the peer cannot agree on channel
 parameters (funding limits, channel reserves, fees, etc.).