waitsendpay: indicate which channel direction the error was.

You can figure this yourself by knowing the route, but it's better to report
it directly here.

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

wallet/db.c

@@ -354,6 +354,8 @@ char *dbmigrations[] = {
     ", state INTEGER"
     ", UNIQUE(in_htlc_id, out_htlc_id)"
     ");",
+    /* Add a direction for failed payments. */
+    "ALTER TABLE payments ADD faildirection INTEGER;", /* erring_direction */
     NULL,
 };