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/uri-js/dist/esnext/schemes/https.d.ts

@@ -0,0 +1,3 @@
+import { URISchemeHandler } from "../uri";
+declare const handler: URISchemeHandler;
+export default handler;