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/astral-regex/index.js

@@ -0,0 +1,4 @@
+'use strict';
+const regex = '[\uD800-\uDBFF][\uDC00-\uDFFF]';
+
+module.exports = opts => opts && opts.exact ? new RegExp(`^${regex}$`) : new RegExp(regex, 'g');