Catch all OS errors when removing old local backups

Errors other than a missing directory or denied permission (e.g. read-only filesystem, device busy) ended in a traceback after the backup.

tests/test_local.py

@@ -1,5 +1,6 @@
 """Tests for local backups, with real rsync runs and with a fake Popen"""
 
+import errno
 import io
 import os
 from collections.abc import Callable, Iterator
@@ -161,6 +162,19 @@ class TestRsync:
         # Nothing to link against, as the old backup was removed first
         assert copied(out, backup, src / 'same.txt').stat().st_nlink == 1
 
+    def test_removal_os_error(self, src: Path, out: Path, monkeypatch: pytest.MonkeyPatch,
+                              caplog: pytest.LogCaptureFixture) -> None:
+        assert run_backup(out, [str(src)]) == 0
+
+        def busy(path: str) -> None:
+            raise OSError(errno.EBUSY, os.strerror(errno.EBUSY), path)
+
+        monkeypatch.setattr(sb, 'rmtree', busy)
+
+        assert run_backup(out, [str(src)], keep=0) == 0
+        assert backups(out) == ['2026-10-14_10-00-00', '2026-10-14_11-00-00']
+        assert 'Error while removing backup 2026-10-14_10-00-00. Device or resource busy' in caplog.text
+
     def test_stray_files_and_symlinks(self, src: Path, out: Path) -> None:
         assert run_backup(out, [str(src)]) == 0