javascript: ignore errors of attachments pooler

Pooling for attachment status is a background operation. Errors should
not be reported to the user, who didn't even ask for this operation to
take place.

This is why we ignore all errors, whether Javascript exceptions or
network errors.

app/javascript/shared/remote-poller.js

@@ -67,7 +67,9 @@ class RemotePoller {
     let urls = this.urls;
     this.reset();
     for (let url of urls) {
-      ajax({ url, type: 'get' });
+      // Start the request. The JS payload in the response will update the page.
+      // (Errors are ignored, because background tasks shouldn't report errors to the user.)
+      ajax({ url, type: 'get' }).catch(() => {});
     }
   }