NuGet dependency discovery is missing an opportunity to report dependency_file_not_found

[brettfo]

If a project, props, or targets file imports another file that's not in the repo, the discovery command should report dependency_file_not_found. This can be detected by scanning the console for message like this:

error MSB4019: The imported project "/some-dir/FileThatDoesNotExist.props" was not found. Confirm that the expression in the Import declaration "/some-dir/FileThatDoesNotExist.props" is correct, and that the file exists on disk.

Currently no error is reported which lets the update job get further where the error is reported, but we should stop early if possible.