feat: introduce new type of error

After pruning is finished + backward sync is introduced, our ErrNoFound will have 2 meanings:
1) The Node is still syncing and hasn't synced to the requested height yet;
2) The node does not have this height and will not sync on it in the future;

As it will confuse us and users, we should agree and differentiate these two cases by introducing one more error for the first case -> `ErrNotSynced`.

cc @renaynay

Start by locating the existing ErrNoFound definition and its call sites in the Go library. Trace how the two described conditions are currently reported, then confirm that the syncing case is distinguishable as ErrNotSynced while the missing-height case retains ErrNoFound.

Written by the indexing model from the issue text.