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/_baseHasIn.js

@@ -0,0 +1,13 @@
+/**
+ * The base implementation of `_.hasIn` without support for deep paths.
+ *
+ * @private
+ * @param {Object} [object] The object to query.
+ * @param {Array|string} key The key to check.
+ * @returns {boolean} Returns `true` if `key` exists, else `false`.
+ */
+function baseHasIn(object, key) {
+  return object != null && key in Object(object);
+}
+
+module.exports = baseHasIn;