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/unary.js

@@ -0,0 +1,22 @@
+var ary = require('./ary');
+
+/**
+ * Creates a function that accepts up to one argument, ignoring any
+ * additional arguments.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Function
+ * @param {Function} func The function to cap arguments for.
+ * @returns {Function} Returns the new capped function.
+ * @example
+ *
+ * _.map(['6', '8', '10'], _.unary(parseInt));
+ * // => [6, 8, 10]
+ */
+function unary(func) {
+  return ary(func, 1);
+}
+
+module.exports = unary;