Satisfy new govet errors

[majst01]

Without this, these errors are reported:

cmd/metal-api/main.go:960:83: printf: non-constant format string in call to fmt.Errorf (govet)
                err := response.WriteAsJson(httperrors.NewHTTPError(serviceErr.Code, fmt.Errorf(serviceErr.Message)))
                                                                                                ^
cmd/metal-api/internal/service/machine-service_allocation_test.go:229:43: printf: non-constant format string in call to fmt.Errorf (govet)
                return v1.MachineResponse{}, fmt.Errorf(w.Body.String())
                                                        ^
cmd/metal-api/internal/service/machine-service_allocation_test.go:251:43: printf: non-constant format string in call to fmt.Errorf (govet)
                return v1.MachineResponse{}, fmt.Errorf(w.Body.String())