add helpful suggestion to identify problem

caddy/sigtrap_posix.go

@@ -24,7 +24,7 @@ func init() {
 			caddyfileMu.Lock()
 			if caddyfile == nil {
 				// Hmm, did spawing process forget to close stdin? Anyhow, this is unusual.
-				log.Println("[ERROR] SIGUSR1: no caddyfile to reload")
+				log.Println("[ERROR] SIGUSR1: no caddyfile to reload (was stdin left open?)")
 				caddyfileMu.Unlock()
 				continue
 			}