backup: revert to catching fewer exceptions

We specifically don't want to catch BrokenPipeError; just list
file-related ones that we might expect to see if we hit bad
permissions, disk errors, or race conditions.

backup.py

@@ -219,7 +219,10 @@ class Backup:
                     for entry in it:
                         self.scan(path=entry.path, parent_st=st)
 
-        except OSError as e:
+        except (FileNotFoundError,
+                IsADirectoryError,
+                NotADirectoryError,
+                PermissionError) as e:
             self.log('E', f"can't read {pstr(path)}: {str(e)}")
             return