Netkan warning for multiple assets

[HebaruSan]

Background

#3279 made it possible to inflate multiple assets per GitHub release, which KSP-CKAN/NetKAN#8338 then enabled for Kopernicus.

Motivations

• There might be other mods out there that could benefit from this, either now or in the future
• Mods that upload mulitple assets may be generating bad metadata if their authors expected Support indexing mulitple release assets on GitHub  #3279 to exist before it existed

Changes

Now if Netkan notices a multi-asset release and isn't configured for it, it prints a warning.