C++/CLI better warning for target pack does not exist in SDK

[wli3]

If build netcoreapp3.1 using master (5.0.1xx) SDK. Since C++/CLI does not support nuget, it will not download the targeting pack. And the error is just. We need a better error

"C:\Users\wul\Downloads\BuildAssetNet---B645529A\NETCoreCppCliTestC\NETCoreCppCliTestC.vcxproj" (default target) (1) ->
(ResolveTargetingPackAssets target) ->
  C:\work\sdk2\artifacts\bin\Debug\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.FrameworkReferenceResolution.targets(283,5): error NETSDK1073: The FrameworkReference 'Microsoft.NETCore.App' was not recognized [C:\Users\wul\Downloads\BuildAssetNet---B645529A\NETCoreCppCliTestC\NETCoreCppCliTestC.vcxproj]