Operation Progress never ends, when nuget restore failed for some projects

I saw that in the latest 17.2 branch build. Actually when I open the project-system repo:

1. Somehow NuGet restore failed, due to:

> NU1301 Unable to load the service index for source https://dotnet.myget.org/F/roslyn-tools/api/v3/index.json. Microsoft.VisualStudio.ProjectSystem.Managed.UnitTests

This seems to be a regression inside IDE. In the command line, I can run msbuild /t:restore correctly.

2. When that failed, operation progress will never end for the solution.

This appears to be a regression. Operation Progress should be completed even nuget store failed. As long as there is no more background work to complete operation progress in the future, it has to be completed, even if it runs into errors. This used to work, and it appears to be a regression.