Add a retry step to fetching features.

[cmunk]

At my workplace we use jbrowse, but get UI errors when fetching features due to status:0 -- it is not a CORS related thing and it occurs randomly and sometimes not at all.

We implemented something like the code in this pull request to allow a single retry and it works in our cases:

These can fail if many with status 0 and a retry succeeds.

If it's relevant for your code then please incorporate it -- or perhaps make a "cleaner" way of retrying failed attempts -- especially for status: 0 -- for us it seems the browser "gives up" on certain requests.

[rbuels]

This is a great idea! Current code looks like it would retry forever if the fetch never succeeded, do you think it would be a good idea to limit the number of retries before failing permanently?

[cmunk]

I think it depends on the status code of the request. Generally I do not think it's every wise to retry past a few attempts as the likely hood of it succeeding falls with each failed attempt :) Ideally an optimal config to allow all requests to be retried x times.

We only saw the issue for features, so I came up with the "hacky" code above. There might be much simpler ways of allowing a retry and definitely a more elegant solution -- but we are Agile and this works ;-)