tox list can consume ValueErrors raised during config discovery

[chillipino]

Issue

when running tox list, the config file discovery code in tox.config.source.discover may semi-silently consume ValueError exceptions that would be useful to report to the user. i say "semi-silently" because no traceback is displayed, but it does print a missing config file warning. my repro is on windows, but i'd expect it's not dependent on OS.

tox run will report that tomllib.TOMLDecodeError: Unescaped '\' in a string is raised for the provided repro case. tomllib.TOMLDecodeError does derive from ValueError.

Environment

Provide at least:

• OS: windows
• python 3.11.3
• pip list of the host Python where tox is installed:

Package       Version
------------- -------
cachetools    5.3.1
chardet       5.1.0
colorama      0.4.6
distlib       0.3.6
filelock      3.12.0
packaging     23.1
pip           22.3.1
platformdirs  3.5.1
pluggy        1.0.0
pyproject_api 1.5.1
setuptools    65.5.0
tox           4.6.0
virtualenv    20.23.0

Output of running tox

Provide the output of tox -rvv list:

ROOT: No tox.ini or setup.cfg or pyproject.toml found, assuming empty tox.ini at D:\bugtemp\pkg
default environments:
py -> [no description]

Minimal example

this pyproject.toml will repro (no other files are necessary):

[tool.tox]
legacy_tox_ini = """

[testenv]
base_python = c:\path\with\unescaped\backslashes

"""

[gaborbernat]

PR welcome.