Improve error message when pushing a NuGet symbol package results in a 404

[loic-sharma]

The NuGet protocol returns a HTTP 404 response status code if the uploaded NuGet symbols package does not correspond to an already existing NuGet package:

This behavior is documented here: https://docs.microsoft.com/en-us/nuget/api/symbol-package-publish-resource#response

The current error message is confusing (see loic-sharma/BaGet#299), we should improve it to state something like A corresponding package with the provided ID and version does not exist.

[diegoberges]

Hello @loic-sharma, I think that it is a good solution for the users