Tell the user if given an invalid / non-existant config file (fix #29)

main.go

@@ -82,6 +82,13 @@ func main() {
 		return
 	}
 
+	if exists, err := fileExists(*config_path); !exists {
+		fmt.Printf("Invalid config file '%s'\n", *config_path)
+		os.Exit(1)
+	} else if err != nil {
+		panic(err)
+	}
+
 	parseConfig(*config_path, &configuration) // Parse JSON configuration file
 
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) { // Webhook receiver