Enumerate all the HTTP errors a request should be retried on.

The python-couchdb module has the following defined, and retries the request if any one of these errors is hit.

```
RETRYABLE_ERRORS = frozenset([
    errno.EPIPE, errno.ETIMEDOUT,
    errno.ECONNRESET, errno.ECONNREFUSED, errno.ECONNABORTED,
    errno.EHOSTDOWN, errno.EHOSTUNREACH,
    errno.ENETRESET, errno.ENETUNREACH, errno.ENETDOWN
])
```
