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>

common/jsonrpc_errors.h

@@ -40,6 +40,7 @@
 #define FUND_CANNOT_AFFORD              301
 #define FUND_OUTPUT_IS_DUST             302
 #define FUNDING_BROADCAST_FAIL          303
+#define FUNDING_STILL_SYNCING_BITCOIN   304
 
 /* Errors from `invoice` command */
 #define INVOICE_LABEL_ALREADY_EXISTS	900