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/lodash/_mapCacheGet.js

@@ -0,0 +1,16 @@
+var getMapData = require('./_getMapData');
+
+/**
+ * Gets the map value for `key`.
+ *
+ * @private
+ * @name get
+ * @memberOf MapCache
+ * @param {string} key The key of the value to get.
+ * @returns {*} Returns the entry value.
+ */
+function mapCacheGet(key) {
+  return getMapData(this, key).get(key);
+}
+
+module.exports = mapCacheGet;