Improve error message when git is not found

[emillon]

There are several cases where a build just aborts with git: command not found. This is fairly unhelpful because:

• it does not explain what causes git to be called (ex: dune subst or dune-build-info with no version in dune-project)
• it does not suggest a solution (like adding a version to dune-project; though that's not often the right fix)
  This happens in certain CI systems, or on nix.