Add warning when serving a non-existent wiki folder

The warning will be yellow to make it noticeable

core/modules/commands/server.js

@@ -17,7 +17,7 @@ if(!$tw.browser) {
 		fs = require("fs"),
 		url = require("url"),
 		path = require("path"),
-		http = require("http");	
+		http = require("http");
 }
 
 exports.info = {
@@ -278,6 +278,9 @@ var Command = function(params,commander,callback) {
 };
 
 Command.prototype.execute = function() {
+	if(!$tw.boot.wikiTiddlersPath) {
+		$tw.utils.warning("Warning: Wiki folder '" + $tw.boot.wikiPath + "' does not exist");
+	}
 	var port = this.params[0] || "8080",
 		rootTiddler = this.params[1] || "$:/core/save/all",
 		renderType = this.params[2] || "text/plain",