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/_listCacheGet.js

@@ -0,0 +1,19 @@
+var assocIndexOf = require('./_assocIndexOf');
+
+/**
+ * Gets the list cache value for `key`.
+ *
+ * @private
+ * @name get
+ * @memberOf ListCache
+ * @param {string} key The key of the value to get.
+ * @returns {*} Returns the entry value.
+ */
+function listCacheGet(key) {
+  var data = this.__data__,
+      index = assocIndexOf(data, key);
+
+  return index < 0 ? undefined : data[index][1];
+}
+
+module.exports = listCacheGet;