               "UNSET"
                    "ECONOMICAL"
                    "CONSERVATIVE"

Result:
{
  "feerate" : x.x,     (numeric, optional) estimate fee rate in BTC/kB
  "errors": [ str... ] (json array of strings, optional) Errors encountered during processing
  "blocks" : n         (numeric) block number where estimate was found
}

The request target will be clamped between 2 and the highest target
fee estimation is able to return based on how long it has been running.
An error is returned if not eno