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/has-flag/index.js

@@ -0,0 +1,8 @@
+'use strict';
+module.exports = (flag, argv) => {
+	argv = argv || process.argv;
+	const prefix = flag.startsWith('-') ? '' : (flag.length === 1 ? '-' : '--');
+	const pos = argv.indexOf(prefix + flag);
+	const terminatorPos = argv.indexOf('--');
+	return pos !== -1 && (terminatorPos === -1 ? true : pos < terminatorPos);
+};