use an OmniSharp specific message for MSB3644

[filipw]

fixes #2029

Instead of printing the default MSBuild message for MSB3644 which is Windows specific and has been very confusing to the users:

The reference assemblies for {0} were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks

we now change it into a more relevant OmniSharp message on Unix:

This project targets .NET version that requires reference assemblies that do not ship with OmniSharp out of the box (e.g. .NET Framework). The most common solution is to make sure Mono is installed on your machine (https://mono-project.com/download/) and that OmniSharp is started with that Mono installation (e.g. 'omnisharp.useGlobalMono':'always' in C# Extension for VS Code).

[JoeRobich]

Thanks for this change