Catch "The reference assemblies (...) were not found." project loader exception and provide better information

[filipw]

Currently when The reference assemblies for .NETFramework,Version=v4.7.1 were not found. is thrown (typically when trying to open desktop .NET project when running on built-in Mono), it is accompanied by a link to download the reference assemblies for Windows only (the default MSBuild error).

A lot of users, especially Unity users, are overwhelmed by this and do not know how to proceed.

We should catch that exception and provide more relevant entry in the log e.g. detecting if user is on *nix, describing they should install global Mono and so on. This could allow users to self-diagnose and fix the problem.