Display Sign-in popup when authentication issue

refs #27 related MR lomiri-system-settings-online-accounts!31 (closed)

this should fix the use case: " I have changed the online Nextcloud/Owncloud password" and UT don't say anything and no sync manual anymore. And should avoid for some, to get banned..."

popup after sync error:

Test:

sudo ubports-qa install focal_-_PR_sync-monitor_61