cornerstone: add loading spinner + errors

[kousu]

On GitHub, a .pdf or .ipynb had a loading spinner

And if the network is cut or fails for some other reason then after about a minute it times out and says this

We don't have either kind of feedback in #30 at the moment, and NIfTI files can be very large (>50MB sometimes) so it is important to give the user some idea of what is happening.