When processing RPC commands during warmup phase, parse the request object before returning an error so that id value can be used in the response.

Prior to this commit, RPC commands sent during Bitcoin's
warmup/startup phase were responded to with a JSON-RPC error
with an id of null, which violated the JSON-RPC 2.0 spec:

id: This member is REQUIRED. It MUST be the same as the value
of the id member in the Request Object. If there was an error
in detecting the id in the Request object (e.g. Parse
error/Invalid Request), it MUST be Null.