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/identity.js

@@ -0,0 +1,21 @@
+/**
+ * This method returns the first argument it receives.
+ *
+ * @static
+ * @since 0.1.0
+ * @memberOf _
+ * @category Util
+ * @param {*} value Any value.
+ * @returns {*} Returns `value`.
+ * @example
+ *
+ * var object = { 'a': 1 };
+ *
+ * console.log(_.identity(object) === object);
+ * // => true
+ */
+function identity(value) {
+  return value;
+}
+
+module.exports = identity;