scripts: sphinx-pre-install: report broken venv

After distro upgrades, the directory names for python may change.
On such case, the previously-created venv will be broken, and
sphinx-build won't run.

Add a logic to report it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/f117f03555b0636d2be0b68f3a23b1d3f03ccf1d.1656756450.git.mchehab@kernel.org


[jc: fixed typo in warning message]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>