Add Inner Exception messages to `NU1301` error for better diagnostic

[Saibamen]

NuGet Product(s) Involved

NuGet.exe, dotnet.exe, MSBuild.exe

The Elevator Pitch

Please add Inner Exception messages to NU1301 error for better diagnostic.

Right now, it is:
NU1301: Unable to load the service index for source {source}

But we don't have any information about root cause - it is the SSL problem, domain not found (typo in feed URL), authentication issue etc.

Additional Context and Details

No response

[nkolev92]

This seems like a dup of #12530, which should've been fixed in NuGet/NuGet.Client#5993.

What's your tooling version @Saibamen?

The fixes were originally shipped in 6.12, https://learn.microsoft.com/en-us/nuget/release-notes/nuget-6.12.

[Saibamen]

I'm using dotnet restore command inside latest (build with --pull) mcr.microsoft.com/dotnet/sdk:8.0 Linux Docker image.

[nkolev92]

So you are using 8.0.x SDKs, but the issue was first fixed in 9.0.100, so if you upgrade, you'll see this go away.

Closing as a dup.

If for whatever reason you're still having issues after upgrade, we'll be happy to reopen.