Hi,
You have a couple of options here. The first is to try including the package into the workspace by adding it to the 'Additional files to include' NCrunch project-level configuration setting.
The second option is to add an extra condition to the error so that it doesn't fire under NCrunch, for example:
<Error Condition="!Exists('..\..\..\packages\Microsoft.Net.Compilers.1.0.0\build\Microsoft.Net.Compilers.props') and '$(NCrunch)' != '1'" Text="$([System.String]::Format('$(ErrorText)', '..\..\..\packages\Microsoft.Net.Compilers.1.0.0\build\Microsoft.Net.Compilers.props'))" />