tuntap: specify error

Closes #34738.

I don’t want to get into the habit of doing this sort of thing, really.
It’s an inelegant solution, and it’s only a solution I turn to because
the tuntap download is so tiny the delay before kicking into the
failure message is at most a few seconds and consequently tolerable,
but generally, I don’t really like failing in install rather than prior
to that.

This particularly failure has to go under def install because otherwise
it causes the ` brew audit ` to raise an unnecessary/confusing error on
the formula.

I went into some detail in #34738 about why this problem is a bit of a
mare to fix. The most elegant, if cumbersome, solution would be for
tuntap to have its own modified MMOSR with a special error message, but
I’m not sure if the maintainers want another inline requirement, given
they can be quite strange/confusing to new contributors and generally a
bit of a pain to maintain.

But I’m open to alternatives and discussion on this. If anyone has a
better idea, I’ll run with that instead. This is more ‘step 1’ rather
than ‘step 1 and done’.