htlc: save fail message in HTLC.

It's not currently encrypted, but at least you get some idea now why
an HTLC failed.  We (ab)use HTTP error codes for the moment.

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

daemon/Makefile

@@ -22,6 +22,7 @@ DAEMON_SRC :=					\
 	daemon/cryptopkt.c			\
 	daemon/db.c				\
 	daemon/dns.c				\
+	daemon/failure.c			\
 	daemon/feechange.c			\
 	daemon/htlc.c				\
 	daemon/jsonrpc.c			\
@@ -64,6 +65,7 @@ DAEMON_HEADERS :=				\
 	daemon/cryptopkt.h			\
 	daemon/db.h				\
 	daemon/dns.h				\
+	daemon/failure.h			\
 	daemon/feechange.h			\
 	daemon/feechange_state.h		\
 	daemon/htlc.h				\