Raise proper exceptions from Relation::Mapper#one

This is a follow up refactoring for #28
Support injecting tuples into SpecHelper#mock_relation
Cover all mutations for Relation::Mapper#one
Fix veritas integration specs for "finding one object"
Extract private Relation::Mapper#restricted_relation
Limit relation to two tuples for Relation::Mapper#one

This avoids fetching a large number of tuples when
only one is expected. Limits the relation to two
entries as this is the minimal number we need for
reporting meaningful error messages in case more
than one tuple would've been returned.

Introduces Engine::Veritas::Node#ordered which can
be considered a WIP and definitely needs more
thought.
Update Engine::Veritas::Node#sort_by signature

Accept what Veritas::Relation#sort_by accepts