Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/request-promise-native/lib/rp.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var configure = require('request-promise-core/configure/request2'),
+    stealthyRequire = require('stealthy-require');
+
+// Load Request freshly - so that users can require an unaltered request instance!
+var request = stealthyRequire(require.cache, function () {
+    return require('request');
+},
+function () {
+    require('tough-cookie');
+}, module);
+
+
+configure({
+    request: request,
+    PromiseImpl: Promise,
+    expose: [
+        'then',
+        'catch',
+        'promise'
+    ]
+});
+
+
+module.exports = request;