don't try to visualize private google spreadsheets links

[mikesall]

When charted receives a google spreadsheets link that isn't set to shareable, it tries to visualize the error message (resulting in a strange chart that makes no sense). Instead, it should provide an error notification saying that the link needs to be set to shareable. (It's worth also checking if there's a similar issue with dropbox links)

[elinw]

That actually didn't happen to me, I got a notice saying it needed to be shareable.