Handle nbviewer failures more gracefully

[krassowski]

Currently:

• currently the text above says "Here is-non interactive preview" which is incorrect
• the error code is the largest thing on the page so a user might believe binder is broken and just give up

Proposed change

I suspect we could make nbviewer emit a cross-frame message and then show something more appropriate on the binder page like:

"The static preview could not be loaded due to heavy traffic, but don't worry the interactive Binder should be ready soon"

Alternative options

Could also try adding a retry but this might make a bad situation wrt GitHub API limits even worse.

Who would use this feature?

Users of Binder

[manics]

Does this require changes to nbviewer, or can we do it entirely in the BinderHub client?