Having done some research on this, it seems that there used to be this same error, but for ToolsVersion 12, when MSFT changed their MSBuild tools in VS '13. It looked like at the time the problem was resolved on the Windows side, but I could never find a solution for OS X or Linux.
I have created a solution within VS '15 that contains a PCL project, with the Windows App Store, Windows Phone 8, Xamarin iOS and Xamarin Android profiles set. Then checked it from Windows in to Git and tried to open it on OS X with Xamarin Studio. I received the aforementioned error.
I have seen that a work-around seems to be setting the ToolsVersion to 4.0 in the .csproj file. If I did that, how does that affect the project file going back in to VS '15 when I pull the changes down from git? Is there a work-around available that does not have me creating a new solution/project for use within Xamarin? There are 11 projects in the solution, half of which are PCL projects. Trying to keep new files and file relocations in sync between the two solutions would be a real pain and error prone.
Any guidance on this would be greatly appreciated!