Add --fatal-meson-warnings to meson dist

[matthiasclasen]

I am currently stuck trying to figure out why meson dist tells me "Dist currently only works with Git or Mercurial repos" when I run it in a git checkout in gitlab ci. The code around that unhinged comment in mdist.py does a few mlog.warning calls, but those never make it anywhere I could use them to find out what it doesn't like about the git checkout. This is very frustrating. If I could set --fatal-meson-warnings, or --verbose or somesuch, maybe I would not be this stuck.

[matthiasclasen]

I also think that the error message is extremely poor. I was in a git repository, and your code is eating the one helpful error message (from git ls-files), only to tell me "Dist only works in a git repository". This cost me hours, since it only happens in ci. I would strongly recommend to prove the error reporting around this, to avoid angry users.