Adds 7 new tests in TestErrorRecovery class covering:
- Hardlink failure cleanup (incomplete backup removal)
- Rsync failure cleanup
- Detection of incomplete backups without marker files
- Lock handling after backup failures
- Permission error handling during cleanup
Tests verify that failed backups are properly cleaned up and don't interfere
with subsequent backup operations or lock management.
Maks Snegov
52b4fef814