[git][editor] Handle cases gracefully when trying to open an unsupported resource

[kittaakos]

For instance when programmatically trying to open URIs pointing to a directory.

When cloning a new repository into an existing repository, Git gives back an FS path to the root of the new repository. It should not be possible to open that directory in an editor. Either do nothing (but log the attempt as a warning) or follow the VSCode's path and open an editor with some text indicating that it is not supported.

This task is a follow-up task of this.