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/nice-try/src/index.js

@@ -0,0 +1,12 @@
+'use strict'
+
+/**
+ * Tries to execute a function and discards any error that occurs.
+ * @param {Function} fn - Function that might or might not throw an error.
+ * @returns {?*} Return-value of the function when no error occurred.
+ */
+module.exports = function(fn) {
+
+	try { return fn() } catch (e) {}
+
+}