Two changes:
1. The HTTP retry path has been refactored to make retries on errors
that are not RestClient exceptions possible by bringing the logic up a
level. This also has the effect of somewhat simplifying the exception
handling logic which can be somewhat difficult to reason about right
now.
2. Retry on `RestClient::Conflict` (a 409 from the API) as discussed in
issue #431.
Fixes#431.
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