Catch unknown command error

modules/hello_world/index.js

@@ -24,4 +24,5 @@ let reply = {
         }
     }
 };
+
 console.log(JSON.stringify(reply));

src/module_manager.js

@@ -1,6 +1,11 @@
 const fs = require("fs");
 const Module = require("./module");
 
+const UNKNOWN_MESSAGE = {
+    format: "standard",
+    message: "Unkown command"
+}
+
 class ModuleManager {
     constructor() {
         console.log("Creating module manager");
@@ -24,7 +29,15 @@ class ModuleManager {
     }
 
     execute(prefix, message) {
-        let rawResult = this.moduleList[prefix].run(message);
+        let module = this.moduleList[prefix];
+
+        if(module === undefined) {
+            this.reply(message, UNKNOWN_MESSAGE);
+
+            return;
+        }
+
+        let rawResult = module.run(message);
         let result = JSON.parse(rawResult.toString());
 
         if(result.status === "success") {