Default behavior when an entity is not found

[igamigo]

For some store methods (and that are also surfaced in Client APIs), we are returning errors when a specific ID is not found. For example, searching for an account ID or a single Note ID results in an error.
A perhaps more usable alternative could be to return an Option<> that determines whether the entity was found or not.

[igamigo]

Related:

Also, the behavior of NoteFilter::Unique seems a bit unintuitive to me. Specifically, why not return an empty vector if the note with the specified ID could not be found? This would be much more consistent with the behavior of all other filters. Let's create an issue to discuss this.

Originally posted by @bobbinth in #508 (comment)