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/p-try/index.js

@@ -0,0 +1,9 @@
+'use strict';
+
+const pTry = (fn, ...arguments_) => new Promise(resolve => {
+	resolve(fn(...arguments_));
+});
+
+module.exports = pTry;
+// TODO: remove this in the next major version
+module.exports.default = pTry;