Show error message when statefile fails to load

[fonsp]

You can test this PR by clicking on a featured notebook, and then editing the URL to make the statefile a bad URL.

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="statefile-load-error-message")
julia> using Pluto

[fonsp]

@pankgeorg this is probably compatible with JH but you might want to make sure. It just adds a .catch to the promise returned by your overloaded get_statefile function.