intersphinx: Stringify error arguments in _display_failures

[cobaltt7]

Purpose

I got this error when running sphinx-build:

    Traceback (most recent call last):
      File "...\black\.venv\Lib\site-packages\sphinx\ext\intersphinx\_load.py", line 235, in _display_failures
        formatted.append(failure_args[0] % failure_args[1:])
                         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
    TypeError: unsupported operand type(s) for %: 'int' and 'tuple'

It looks like something threw this error:

OSError: [Errno 22] Invalid argument: '...\\black\\docs\\_build\\.doctrees\\__intersphinx_cache__\\<name>_objects.inv'

And err.args was (22, "Invalid argument"). When it was passed to _display_failures, it failed to generate a proper error string.
I'm still investigating the cause of the OSError, but for now¹ Sphinx should be fixed to propagate the error message properly.

I didn't add a changelog entry because it's still the 9.1.0 changelog - should I be the one to update it, or should I wait for someone else?

Docutils 0.22 on py312 and ty typecheck were both failing on main already.
Docutils failure on py315 doesn't seem to be my fault either - it's fixed in #14399

References

https://github.com/psf/black/actions/runs/22561832229/job/65349935011 - CI log that first alerted me to the error

Full traceback

Versions
========

* Platform:         win32; (Windows-11-10.0.26200-SP0)
* Python version:   3.12.4 (CPython)
* Sphinx version:   9.1.0
* Docutils version: 0.22.4
* Jinja2 version:   3.1.6
* Pygments version: 2.19.2

Last Messages
=============

None.

Loaded Extensions
=================

None.

Traceback
=========

    Traceback (most recent call last):
      File "...\black\.venv\Lib\site-packages\sphinx\ext\intersphinx\_load.py", line 235, in _display_failures
        formatted.append(failure_args[0] % failure_args[1:])
                         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
    TypeError: unsupported operand type(s) for %: 'int' and 'tuple'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "...\black\.venv\Lib\site-packages\sphinx\events.py", line 441, in emit
        results.append(listener.handler(self._app, *args))
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "...\black\.venv\Lib\site-packages\sphinx\ext\intersphinx\_load.py", line 191, in load_mappings
        updated = [f.result() for f in concurrent.futures.as_completed(futures)]
                   ^^^^^^^^^^
      File "C:\Program Files\Python312\Lib\concurrent\futures\_base.py", line 449, in result
        return self.__get_result()
               ^^^^^^^^^^^^^^^^^^^
      File "C:\Program Files\Python312\Lib\concurrent\futures\_base.py", line 401, in __get_result
        raise self._exception
      File "C:\Program Files\Python312\Lib\concurrent\futures\thread.py", line 58, in run
        result = self.fn(*self.args, **self.kwargs)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "...\black\.venv\Lib\site-packages\sphinx\ext\intersphinx\_load.py", line 333, in _fetch_inventory_group
        _display_failures(failures),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "...\black\.venv\Lib\site-packages\sphinx\ext\intersphinx\_load.py", line 237, in _display_failures
        formatted.append(' - '.join(failure_args))
                         ^^^^^^^^^^^^^^^^^^^^^^^^
    TypeError: sequence item 0: expected str instance, int found

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "...\black\.venv\Lib\site-packages\sphinx\cmd\build.py", line 414, in build_main
        app = Sphinx(
              ^^^^^^^
      File "...\black\.venv\Lib\site-packages\sphinx\application.py", line 340, in __init__
        self._init_builder()
      File "...\black\.venv\Lib\site-packages\sphinx\application.py", line 430, in _init_builder
        self.events.emit('builder-inited')
      File "...\black\.venv\Lib\site-packages\sphinx\events.py", line 452, in emit
        raise ExtensionError(
    sphinx.errors.ExtensionError: Handler <function load_mappings at 0x00000192CA9C8860> for event 'builder-inited' threw an exception (exception: sequence item 0: expected str instance, int found)

Footnotes

1. An intersphinx_mapping key had </> in it. When Sphinx tried to use it as part of a cache file name, it crashed on Windows, which doesn't allow them in file names. This is more user error than Sphinx's fault, but it should have a clearer error message. ↩