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/symbol-tree/lib/TreePosition.js

@@ -0,0 +1,11 @@
+'use strict';
+
+/* eslint-disable sort-keys */
+module.exports = Object.freeze({
+        // same as DOM DOCUMENT_POSITION_
+        DISCONNECTED: 1,
+        PRECEDING: 2,
+        FOLLOWING: 4,
+        CONTAINS: 8,
+        CONTAINED_BY: 16,
+});