Package creation step reports success even when it fails

[idg10]

Becaused of endjin/Endjin.RecommendedPractices.NuGet#20 we are currently producing files named .xml. These in turn cause failures during package creation, e.g.:

/opt/hostedtoolcache/dotnet/sdk/3.1.401/Sdks/NuGet.Build.Tasks.Pack/build/NuGet.Build.Tasks.Pack.targets(198,5): error NU5119: File '/home/vsts/work/1/s/Solutions/Corvus.Azure.Cosmos.Tenancy/bin/Release/netstandard2.0/.xml' was not added to the package. Files and folders starting with '.' or ending with '.nupkg' are excluded by default. To include this file, use -NoDefaultExcludes from the commandline [/home/vsts/work/1/s/Solutions/Corvus.Azure.Cosmos.Tenancy/Corvus.Azure.Cosmos.Tenancy.csproj]

You can see an example of this failure here:

https://dev.azure.com/endjin-labs/Corvus.Tenancy/_build/results?buildId=6365&view=logs&j=ca395085-040a-526b-2ce8-bdc85f692774&t=bde858e3-1ff6-5d7a-f958-9ce4fc5e6086&l=1356

But notice that the build step containing this failure appears to have succeeded. Indeed, the whole build looks to have succeeded
https://dev.azure.com/endjin-labs/Corvus.Tenancy/_build/results?buildId=6365&view=results

even though it has failed to build NuGet packages correctly, an important part of the build.

Errors during this step should be reflected in the build status.