Accept singular pdb exception command - #15206

The remaining failing check is the downstream test (macos-14, 3.13) job. The failure is in pyflyby tests/test_interactive.py::test_error_during_completion_1[prompt_toolkit], where the pty transcript differs around completion/error output.

This PR only adds the singular exception debugger command alias under the existing chained-exception command path; that path is skipped on Python 3.13, and the Ubuntu 3.13 downstream job passed. I tried rerunning the failed job, but GitHub requires repository admin permissions for that action.

thanks