MvcBuildViews fails with a cryptic MSB4803 under the dotnet CLI (AspNetCompiler is unavailable)

[ehasis]

Split out from #103 (item 3).

Summary

In Release the SDK sets MvcBuildViews=true and runs the AspNetCompiler task in the MvcBuildViews target. That task ships only with the full .NET Framework MSBuild, so under dotnet (Core MSBuild) the build fails with:

error MSB4803: The task "AspNetCompiler" is not supported on the .NET Core version of MSBuild.

Proposed change

Under Core MSBuild, replace the cryptic MSB4803 with a clear, actionable message (e.g. SYSWEB002) that explains AspNetCompiler is unavailable on the .NET Core MSBuild used by dotnet, and that the step can be skipped intentionally by setting:

<!-- e.g. in Directory.Build.props -->
<MvcBuildViews Condition="'$(MSBuildRuntimeType)' == 'Core'">false</MvcBuildViews>

Skipping stays opt-in (the user's explicit choice); precompiling views with the full Visual Studio MSBuild is unchanged.

(Open to either: a hard error with the guidance above, or a non-fatal message + skip behind an opt-in property — whichever you prefer.)

[CZEMacLeod]

I think I prefer the hard error. There is a mechanism to skip the task already in place e.g. setting MvcBuildViews to false.
Adding an opt-in property to not set MvcBuildViews seems like overkill.
I tend to have it always disabled in my projects as I use RazorCompiler and don't need any AspNetCompiler step in my main project.
Others may want it to run always (e.g. in Debug and Release) and set it unconditionally to true.
Adding a conditional line on MSBuildRuntimeType seems like the best approach if the user is happy to accept not running the compiler task under the dotnet cli.

See also #109 regarding error messages.