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/_getMapData.js

@@ -0,0 +1,18 @@
+var isKeyable = require('./_isKeyable');
+
+/**
+ * Gets the data for `map`.
+ *
+ * @private
+ * @param {Object} map The map to query.
+ * @param {string} key The reference key.
+ * @returns {*} Returns the map data.
+ */
+function getMapData(map, key) {
+  var data = map.__data__;
+  return isKeyable(key)
+    ? data[typeof key == 'string' ? 'string' : 'hash']
+    : data.map;
+}
+
+module.exports = getMapData;