I had this error too and it was literally the file path that was too long. I post here for the case people encounter the same:
I had my project at:
C:\Users\stevenv\Documents\Visual Studio 2015\Projects\Unity - Technologies - graphicsdemos - 43011994ae74\Unity - Technologies - graphicsdemos - 43011994ae74\NativeRenderingPlugin\PluginSource\projects\VisualStudio2015=220 chars
Moving it too a location with a shorter path solved it.
C:\Users\stevenv\Documents\Visual Studio 2015\Projects\RenderPluginDemo\NativeRenderingPlugin\PluginSource\projects\VisualStudio2015
=133 chars
So in 2017 we still have to deal with these problems?