This is a Docker compatibility change discovered while working on
cases it is practically speaking required because there are
mandatory parameters in the body, but in those cases you do not
get a JSON decode error back, you get an error about the field
that needs to be set. I see no reason for us not to match this
convention; it doesn't break our existing bindings, but makes
using the API via curl or similar somewhat easier.
Signed-off-by: Matthew Heon <matthew.heon@pm.me>