build.ps1 - Remove incorrect $env:VS141COMNTOOLS -eq $null check when building VS2017
build.ps1 - Remove incorrect $env:VS141COMNTOOLS -eq $null check when building VS2017

There is no such environment variable, VS2017 uses vswhere to locate the installed version.

Move checks into the MSVS function and make the error message consistent

Livit/CefSharp,Livit/CefSharp,Livit/CefSharp,Livit/CefSharp