fix crash if previous event file does not exist

src/scrape.js

@@ -177,11 +177,13 @@ const { page_ids, output, events: event_file } = parse_args(
 
 const read_previous_events = (path) => {
   if (path !== null) {
+    if (filesystem.existsSync(path)) {
       return fs
         .readFile(path, { encoding: 'utf-8' })
         .then((content) => JSON.parse(content))
         .catch(console.error);
     }
+  }
   return Promise.resolve([]);
 };