Errors in output window do not show up in error list

I am porting a .NET Core project (built using csproj from directions in https://docs.microsoft.com/en-us/dotnet/articles/core/tutorials/target-dotnetcore-with-msbuild) to VS2017. Since the migration tool does not work for csproj, I am doing it manually. I am running into cases where I can see there are errors in the Output Window but not the Error Window.

Repro.
1. git clone https://github.com/conniey/icu-dotnet
2. git fetch
3. git checkout vs2017noerrors
4. Download the attached nupkg... because I modified my NuGet.config to accept that nupkg from a local directory so you'll have to modify that as well.
5. Open source\icu.net.sln
6. Unload all the projects except: "icu.net.netstandard"
7. Right-click and edit it "Edit icu.net.netstandard.csproj"
8. Rebuild solution from Solution Explorer.

Expected: Errors in the Error List and Output Window match
Actual: I have to look at the output window and the build has failed but shows nothing.