fix relative_to on internal paths

[cebtenzzre]

If a NameError or ImportError is thrown inside of appdaemon, the relative_to will fail like this:

Traceback (most recent call last):
  File "/usr/lib/python3.12/site-packages/appdaemon/utility_loop.py", line 246, in _loop_iteration_context
    yield timing
  File "/usr/lib/python3.12/site-packages/appdaemon/utility_loop.py", line 197, in loop
    await self.AD.app_management.check_app_updates()
  File "/usr/lib/python3.12/site-packages/appdaemon/app_management.py", line 930, in check_app_updates
    await self._create_and_start_apps(update_actions)
  File "/usr/lib/python3.12/site-packages/appdaemon/app_management.py", line 1222, in _create_and_start_apps
    await safe_start(self)
  File "/usr/lib/python3.12/site-packages/appdaemon/exceptions.py", line 182, in wrapper
    user_exception_block(logger, e, app_dir, header)
  File "/usr/lib/python3.12/site-packages/appdaemon/exceptions.py", line 116, in user_exception_block
    file = Path(frame.filename).relative_to(app_dir.parent)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/pathlib.py", line 682, in relative_to
    raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")

This change causes that text to only be appended for exceptions thrown in user app code.

Related to #2457