new: fallback host support for 503 errorNum

[aMahanna]

From a discussion with the ArangoDB Java & Go maintainers (cc @rashtao):

please note that failover/retry in case of status 503 should happen only in these cases:
1. the response has body like {"error":true,"errorNum":503,"code":503, ...}, note errorNum=503
2. the 503 response comes from an intermediate proxy, so it does not contain a json body like the one above
Other 503 responses with different errorNum should not lead to failover/retry, but just thrown to the caller.