Improved error handling for Dependabot updates for NuGet

Dependabot will now fail gracefully with informative error messages when an unsupported NuGet project type is encountered. If you were using an unsupported project type previously, Dependabot might have failed silently without producing updates. Dependabot is able to process updates to NuGet project files in the .csproj, .vbproj, and .fsproj formats.