Add cmake error message when cmake can not find shray.h

I had some problems with shray not being there when after I cloned it. This merge request gives you a better error message when this happens and suggests a way to fix your problem. It gives you the message:

shray.h header file does not exist. It should be in ${PROJECT_SOURCE_DIR}/ext/shray2/include/shray2/
Try running: git submodule update --init --recursive