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/istanbul-lib-source-maps/index.js

@@ -0,0 +1,15 @@
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+'use strict';
+
+const { MapStore } = require('./lib/map-store');
+/**
+ * @module Exports
+ */
+module.exports = {
+    createSourceMapStore(opts) {
+        return new MapStore(opts);
+    }
+};