Issue with precondition_error handling

[aslafy-z]

While trying to debug #2603, I got a backend error that was not correctly handled by the SDK:

{
    "message": "Cannot create a volume of type sbs_volume from a base of type l_ssd",
    "type": "precondition_failed"
}

From what I understand here:

• https://github.com/scaleway/scaleway-sdk-go/blob/master/scw/errors.go#L150-L151
• https://github.com/scaleway/scaleway-sdk-go/blob/master/scw/errors.go#L542-L567

Precondition errors should have a precondition and optionally a help_message. However, these two fields are absent in the backend response.