backup: catch all OSError exceptions while accessing files

We might see these if files change during the scan, for example.

backup.py

@@ -219,8 +219,8 @@ class Backup:
                     for entry in it:
                         self.scan(path=entry.path, parent_st=st)
 
-        except PermissionError as e:
-            self.log('E', f"can't read {pstr(path)}")
+        except OSError as e:
+            self.log('E', f"can't read {pstr(path)}: {str(e)}")
             return
 
 def main(argv: list[str]):