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/lowerFirst.js

@@ -0,0 +1,22 @@
+var createCaseFirst = require('./_createCaseFirst');
+
+/**
+ * Converts the first character of `string` to lower case.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category String
+ * @param {string} [string=''] The string to convert.
+ * @returns {string} Returns the converted string.
+ * @example
+ *
+ * _.lowerFirst('Fred');
+ * // => 'fred'
+ *
+ * _.lowerFirst('FRED');
+ * // => 'fRED'
+ */
+var lowerFirst = createCaseFirst('toLowerCase');
+
+module.exports = lowerFirst;