don't offer to generate an embed link if the project couldn't be embedded

[Dan Bean]

users could assume the embed link will always embed whatever is displayed, but that's only true if it's saved or generated automatically (and not modified). E.g. if I start with an empty network, then upload a list and click "generate embed link", it will just point to an empty network which might be unexpected behaviour. We should just replace that popup with a message until the graph is saved.