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/escape-string-regexp/index.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var matchOperatorsRe = /[|\\{}()[\]^$+*?.]/g;
+
+module.exports = function (str) {
+	if (typeof str !== 'string') {
+		throw new TypeError('Expected a string');
+	}
+
+	return str.replace(matchOperatorsRe, '\\$&');
+};