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/w3c-hr-time/lib/utils.js

@@ -0,0 +1,11 @@
+"use strict";
+
+// Browserify's process implementation doesn't have hrtime, and this package is small so not much of a burden for
+// Node.js users.
+const hrtime = require("browser-process-hrtime");
+
+function toMS([sec, nanosec]) {
+  return sec * 1e3 + nanosec / 1e6;
+}
+
+module.exports = { hrtime, toMS };