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/urn.d.ts

@@ -0,0 +1,10 @@
+import { URISchemeHandler, URIComponents, URIOptions } from "../uri";
+export interface URNComponents extends URIComponents {
+    nid?: string;
+    nss?: string;
+}
+export interface URNOptions extends URIOptions {
+    nid?: string;
+}
+declare const handler: URISchemeHandler<URNComponents, URNOptions>;
+export default handler;