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/mailto.d.ts

@@ -0,0 +1,12 @@
+import { URISchemeHandler, URIComponents } from "../uri";
+export interface MailtoHeaders {
+    [hfname: string]: string;
+}
+export interface MailtoComponents extends URIComponents {
+    to: Array<string>;
+    headers?: MailtoHeaders;
+    subject?: string;
+    body?: string;
+}
+declare const handler: URISchemeHandler<MailtoComponents>;
+export default handler;