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/urix/index.js

@@ -0,0 +1,17 @@
+// Copyright 2014 Simon Lydell
+// X11 (“MIT”) Licensed. (See LICENSE.)
+
+var path = require("path")
+
+"use strict"
+
+function urix(aPath) {
+  if (path.sep === "\\") {
+    return aPath
+      .replace(/\\/g, "/")
+      .replace(/^[a-z]:\/?/i, "/")
+  }
+  return aPath
+}
+
+module.exports = urix