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/hasIn.js

@@ -0,0 +1,34 @@
+var baseHasIn = require('./_baseHasIn'),
+    hasPath = require('./_hasPath');
+
+/**
+ * Checks if `path` is a direct or inherited property of `object`.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Object
+ * @param {Object} object The object to query.
+ * @param {Array|string} path The path to check.
+ * @returns {boolean} Returns `true` if `path` exists, else `false`.
+ * @example
+ *
+ * var object = _.create({ 'a': _.create({ 'b': 2 }) });
+ *
+ * _.hasIn(object, 'a');
+ * // => true
+ *
+ * _.hasIn(object, 'a.b');
+ * // => true
+ *
+ * _.hasIn(object, ['a', 'b']);
+ * // => true
+ *
+ * _.hasIn(object, 'b');
+ * // => false
+ */
+function hasIn(object, path) {
+  return object != null && hasPath(object, path, baseHasIn);
+}
+
+module.exports = hasIn;