kubernetes: Server-Side Apply should return a proper http status when validation fails

Server-Side Apply has different constraints for updating the managed fields and applying than typical objects, and sometimes the discrepency can cause problem.

When trying to update the managedFields, if the object doesn’t validate, then the whole managedFields list is removed, the error is ignored as to continue with the normal process. But then such an object can’t ever be applied, since it can’t be converted into the type of objects that server-side apply uses.

Right now, when this happens, the server-side apply request fails with a generic 500. We’re trying decide what code should be returned in that case (solving the problem of applying such an object is a separate issue), so that the client can possibly try a different way. Here’s the problem though: there is fundamentally nothing wrong with the client request, so 4xx doesn’t really fit, and yet, the client should take an action since there isn’t much we can do server-side, so a 5xx doesn’t really fit either.

cc @thockin @alculquicondor @aojea @jpbetz