DOC make_montage():  More descriptive error if make_montage() is called with blank image
DOC make_montage():  More descriptive error if make_montage() is called with blank image

effigies/PySurfer,mwaskom/PySurfer,Eric89GXL/PySurfer,bpinsard/PySurfer,diego0020/PySurfer,nipy/PySurfer,haribharadwaj/PySurfer