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/_getAllKeysIn.js

@@ -0,0 +1,17 @@
+var baseGetAllKeys = require('./_baseGetAllKeys'),
+    getSymbolsIn = require('./_getSymbolsIn'),
+    keysIn = require('./keysIn');
+
+/**
+ * Creates an array of own and inherited enumerable property names and
+ * symbols of `object`.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @returns {Array} Returns the array of property names and symbols.
+ */
+function getAllKeysIn(object) {
+  return baseGetAllKeys(object, keysIn, getSymbolsIn);
+}
+
+module.exports = getAllKeysIn;