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/lodash/escapeRegExp.js

@@ -0,0 +1,32 @@
+var toString = require('./toString');
+
+/**
+ * Used to match `RegExp`
+ * [syntax characters](http://ecma-international.org/ecma-262/7.0/#sec-patterns).
+ */
+var reRegExpChar = /[\\^$.*+?()[\]{}|]/g,
+    reHasRegExpChar = RegExp(reRegExpChar.source);
+
+/**
+ * Escapes the `RegExp` special characters "^", "$", "\", ".", "*", "+",
+ * "?", "(", ")", "[", "]", "{", "}", and "|" in `string`.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.0.0
+ * @category String
+ * @param {string} [string=''] The string to escape.
+ * @returns {string} Returns the escaped string.
+ * @example
+ *
+ * _.escapeRegExp('[lodash](https://lodash.com/)');
+ * // => '\[lodash\]\(https://lodash\.com/\)'
+ */
+function escapeRegExp(string) {
+  string = toString(string);
+  return (string && reHasRegExpChar.test(string))
+    ? string.replace(reRegExpChar, '\\$&')
+    : string;
+}
+
+module.exports = escapeRegExp;