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

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](https://developercommunity.visualstudio.com/content/problem/571032/target-framework-is-ignored-when-generating-intero.html) (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.