Selective retry v2 API requests

[jonmmease]

In version 3 we introduced automatic retrying logic for all v2 REST API calls to plot.ly. This has improved the reliability and reduced the incidence of failures do to intermittent server connectivity issues.

But, we're currently retrying on all requests, even cases where there's really no point (e.g. an error due to an invalid figure URL isn't going to go away by retrying many times).

As a starting point, let's keep retrying on 5XX errors, stop retrying on the 4XX errors, and investigate the nature of the 2XX errors that still occur.

This should hopefully reduce the runtime of our test suite and maintain the reliability of retrying when appropriate.

[jonmmease]

Done in #1146