Default to offline if server error or no internect connection, and raise message

[jackparmer]

Right now, we just hard fail if there is a 504, other server error, or no internet connection. Here's what happens with a 504:

Would be great if we rendered offline instead, with a prominent message that a 504 (or whatever) occurred and how to sync online when service is back.

[jonmmease]

Rather than automatically falling back to offline, the current plan is to work towards unifying the online/offline API, making it much easier to switch back and forth. See discussion of plotly.io.show concept in #1098