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 particular method of 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’.
Similar to [Clutter](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/clutter.rb#L24) and other places.