Allow go idiomatic return of nil MountResponse, PathResponse and GetResponse on error for driver implementations

[fanktom]

An idiomatic go function returns value, nil or nil, error on error.

The current implementation requires that on error the implementer of a driver returns an empty response struct and an error. This PR allows to return an nil, error as one would expect, does not break tests or the API.

Currently:

return &GetResponse{}, errors.New("some error")

After PR:

return nil, errors.New("some error")

Further, the current API still works the same.