File does not exist error improvement

[gytis-ivaskevicius]

Is your feature request related to a problem? Please describe.
When importing untracked nix file error such as is shown: error: path '/nix/store/xxx-source/something.nix' does not exist
I noticed some newbies struggling with it and I think better error message should be shown

Describe the solution you'd like
If it is a git repository it should show something like this:

• error: path '/nix/store/xxx-source/something.nix' exists but is not tracked by git (Might be annoying to implement)
• error: path '/nix/store/xxx-source/something.nix' does not exist or is not tracked by git

Describe alternatives you've considered
None, this is more of a nice to have improvement to make onboarding process easier

Priorities

Add 👍 to issues you find important.

[Sigmanificient]

Maybe it could be possible to retrieve the fullpath and stat the file (or maybe using libgit), to help distinguish the error into 2 separate ones. I think this would be very nice as nix beginners often get confused with this error

[roberth]

IIRC we have a solution that will land as we diligently port changes from lazy trees to master.

Duplicate of #7107