NuGet updater should detect `404` on base package feed

A feed in a `NuGet.Config` file that returns a `404` will cause the following output during dependency discovery:

```
error : Unable to load the service index for source https://not.found.example.com/nuget/v3/index.json. [/path/to/project.csproj]
error : Response status code does not indicate success: 404 ()
```

That message should be explicitly handled to return a specific error message (message TBD.)