Handle rclone sync errors in task results

[scr4bble]

Fixes #407

Summary

This updates sync task result handling so rclone runs that complete with errors are no longer reported as clean successes or as “There was nothing to do”.

The sync result now:

• reports completed-with-errors when final rclone status contains errors
• preserves transfer/deletion/rename summary details even when errors occurred
• only shows “There was nothing to do” when there were no changes and no errors
• avoids duplicating retry-attempt errors in the final notification text
• keeps rclone’s normal safety behavior where destination deletions are skipped after source/listing errors
• persists relevant rclone warning/error log lines from the sync stderr stream
• makes sync notifications open the Logs page
• avoids confusing report notifications by showing only result of the latest task sync in the expanded notification text

[scr4bble]

Adding a few screenshots from testing: