Missing notification when trying to download from unavailable storage

[HanaGemela]

Found after testing #5088

Client: 2.6.0rc1 (build 12411)
mac 10.14.6
server: 10.2.0 stable

Steps to recreate:

1. Start two servers:

docker run -e OWNCLOUD_SHARING_FEDERATION_ALLOW_HTTP_FALLBACK=true -p 8080:8080 owncloud/server
docker run -e OWNCLOUD_SHARING_FEDERATION_ALLOW_HTTP_FALLBACK=true -p 8081:8080 owncloud/server

2. User1 shares a folder with User2 from another server (federated share)
3. Turn off the first server
4. User2 dehydrates the shared file
5. User2 double clicks on the shared file to download it back

Actual result: Nothing
Expected result: Notification with explanation why the file cannot be downloaded

[ogoffart]

So there isn't even a notification in the system tray that the sync failed? Maybe because we don't show error for temporarily unavailable files.

Application::openVirtualFile currently does not report any error, but i guess we should show a QMessageBox if there was an error. We can maybe try to get the error from the SyncResult::_syncItems that we have from the signal that the sync is finished

[michaelstingl]

I'd expect at least an entry in the Activity > Not synced tab. Maybe some kind of notification can be shown somewhere…