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/property.js

@@ -0,0 +1,32 @@
+var baseProperty = require('./_baseProperty'),
+    basePropertyDeep = require('./_basePropertyDeep'),
+    isKey = require('./_isKey'),
+    toKey = require('./_toKey');
+
+/**
+ * Creates a function that returns the value at `path` of a given object.
+ *
+ * @static
+ * @memberOf _
+ * @since 2.4.0
+ * @category Util
+ * @param {Array|string} path The path of the property to get.
+ * @returns {Function} Returns the new accessor function.
+ * @example
+ *
+ * var objects = [
+ *   { 'a': { 'b': 2 } },
+ *   { 'a': { 'b': 1 } }
+ * ];
+ *
+ * _.map(objects, _.property('a.b'));
+ * // => [2, 1]
+ *
+ * _.map(_.sortBy(objects, _.property(['a', 'b'])), 'a.b');
+ * // => [1, 2]
+ */
+function property(path) {
+  return isKey(path) ? baseProperty(toKey(path)) : basePropertyDeep(path);
+}
+
+module.exports = property;