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/ajv/lib/compile/equal.js

@@ -0,0 +1,5 @@
+'use strict';
+
+// do NOT remove this file - it would break pre-compiled schemas
+// https://github.com/epoberezkin/ajv/issues/889
+module.exports = require('fast-deep-equal');