Warn the user when building a project targeting 3.5 but the 3.5 SDK isn't installed

[tmeschter]

When building a project that targets the 2.0/3.0/3.5 Framework we try to use the tools defined by the $(SDK35ToolsPath) property:

https://github.com/microsoft/msbuild/blob/d42d3504057ef2b88dd4f68c4bfc5591371bd6fe/src/Tasks/Microsoft.NETFramework.CurrentVersion.props#L85

which we read from the registry. However, if the 3.5 development tools aren't installed then we won't find the value in the registry, $(SDK35ToolsPath) will not be defined, and we will silently "upgrade" to the 4.x tools instead:

https://github.com/microsoft/msbuild/blob/d42d3504057ef2b88dd4f68c4bfc5591371bd6fe/src/Tasks/Microsoft.NETFramework.CurrentVersion.props#L90

This can cause problems, such as using the wrong version of tlbimp to generate interop assemblies (which subsequently won't load at runtime).

We should instead detect and warn the user (or maybe even emit an error) when the 3.5 tools aren't installed.

[livarcocc]

Any opinions on this qualifying as a breaking change though? Given we would be warning where we previously didn't?

[rainersigwald]

We've made "breaking" changes before where we went from "silently producing bad outputs" to "error at build time" and people seemed happy enough with it. In this case, hopefully few people target 3.5, but those people are increasingly likely to not have 3.5 installed on their new machine. I think it's probably worth adding the error.