Unify status usage in storage engine when object is missing

[carpawell]

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

I'm always frustrated when we have both bool flags that mean the presence of an object in the engine and ErrObjectNotFound. Sometimes it is a flag and sometimes it is an error.

Describe the solution you'd like

It should either be error, or be a flag.

Describe alternatives you've considered

-

Additional context

Sometimes it is ok to have an error and sometimes we only accept a flag, it is unreliable.
As for me, I wouldn't say I like using client statuses (that is where they are placed in SDK) in engines at all.