Accept both PUT and POST in noded

Currently, noded requires PUT, even though the semantics of the RPC
calls do not match a PUT. We change the code accept both PUT and POST,
with the intention to remove the PUT support in a later version.

On the client side, the RPC code is changed to send POST requests.

Additionally, we add a message to the HttpBadRequest exception to make
clear the failure mode (not seeing any error message was what made me
send this patch…). This was the only description-less use of this
exception, by the way.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>