Unpack errors from `pdb.bdb`

Like any `bdb.BdbQuit` that might be relayed from a remote context after
a REPl exit with the `quit` cmd. This fixes various issues while
debugging where it may not be clear to the parent task that the child
was terminated with a purposefully unrecoverable error.

tractor/_exceptions.py

@@ -22,6 +22,7 @@ from __future__ import annotations
 import builtins
 import importlib
 from pprint import pformat
+from pdb import bdb
 import sys
 from types import (
     TracebackType,
@@ -181,6 +182,7 @@ def get_err_type(type_name: str) -> BaseException|None:
         builtins,
         _this_mod,
         trio,
+        bdb,
     ]:
         if type_ref := getattr(
             ns,