Inform user about invalid secret key

Author: Lenni0451

Agent/src/main/java/net/lenni0451/authhook/Agent.java

@@ -16,6 +16,12 @@ public static void premain(final String args, final Instrumentation instrumentat
     private static void hook(final Instrumentation instrumentation) {
         try {
             Map<String, String> config = Config.load();
+            if (config.get(Config.SECRET_KEY).contains(" ")) {
+                System.err.println("Please set a valid secret key in the auth_hook.properties file");
+                System.err.println("It is automatically generated by ViaProxy if you have the AuthHook plugin installed");
+                System.exit(-1);
+            }
+
             instrumentation.addTransformer(new URLRedirector(config), true);
         } catch (Throwable t) {
             System.err.println("An error occurred while starting AuthHook");