client: Clarify error reason when required response body is missing

[cthulhu-rider]

Is your feature request related to a problem? Please describe.

all response payloads are transmitted in the body field. In most cases ay least one body field is required. When the body is missing in the response, client returns missing X field in the response where X is the 1st required field being checked

overall, this is correct: error returns, and field absence follows from the body absence. But these cases are different from the protobuf pov, i'd separate them from each other. This can simplify debug

Describe the solution you'd like

return errors.New("missing body in the response")

Additional context

Client testing