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/@babel/plugin-syntax-object-rest-spread/lib/index.js

@@ -0,0 +1,30 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.default = void 0;
+
+function _helperPluginUtils() {
+  const data = require("@babel/helper-plugin-utils");
+
+  _helperPluginUtils = function () {
+    return data;
+  };
+
+  return data;
+}
+
+var _default = (0, _helperPluginUtils().declare)(api => {
+  api.assertVersion(7);
+  return {
+    name: "syntax-object-rest-spread",
+
+    manipulateOptions(opts, parserOpts) {
+      parserOpts.plugins.push("objectRestSpread");
+    }
+
+  };
+});
+
+exports.default = _default;