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/stubString.js

@@ -0,0 +1,18 @@
+/**
+ * This method returns an empty string.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.13.0
+ * @category Util
+ * @returns {string} Returns the empty string.
+ * @example
+ *
+ * _.times(2, _.stubString);
+ * // => ['', '']
+ */
+function stubString() {
+  return '';
+}
+
+module.exports = stubString;