fetchGit: Don't barf if we can't update our Git clone
fetchGit: Don't barf if we can't update our Git clone

Instead print a warning that we're continuing with the most recently
fetched version.

NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix