handle missing/unavailiable scm in file finder

[jaraco]

In recently supporting a private project, we found that the project would install, but its package-data was missing. The project was relying on setuptools_scm file finders to find those files and include them in the manifest, but when git was not installed, the finder passed silently (by design, per #81). For projects like this, that do rely on the SCM being present, it would be nice to fail with a nice error message.

This project should consider revisiting the choice in #81 or add an option to fail when the SCM is needed but unavailable.

The setuptools docs indicate that a missing tool should not fail, but could log a warning. Perhaps logging a warning is the appropriate thing (or perhaps that's what already happens).

[RonnyPfannschmidt]

@jaraco when the parsing has no result at all we should actually error

[RonnyPfannschmidt]

@jaraco i missunderstood your description - the git file finder incorrectly assumes no git if git is missing which is unrelated to the normal parse handling from #81

we should have a big warning if the git file finder finds a git folder but no git, perhaps even an error

[RonnyPfannschmidt]

this is a breaking change - wil lhave to determine what to do about it

there probably is need to include a magic file that tracks the found files for a fallback file finder - re-engineering that so we can safely fail will take a bit