fix crash if previous event file does not exist

src/scrape.js

@@ -177,10 +177,12 @@ const { page_ids, output, events: event_file } = parse_args(
 
 const read_previous_events = (path) => {
   if (path !== null) {
-    return fs
-      .readFile(path, { encoding: 'utf-8' })
-      .then((content) => JSON.parse(content))
-      .catch(console.error);
+    if (filesystem.existsSync(path)) {
+      return fs
+        .readFile(path, { encoding: 'utf-8' })
+        .then((content) => JSON.parse(content))
+        .catch(console.error);
+    }
   }
   return Promise.resolve([]);
 };