Refactor KeyExists to avoid logging errors when key not found

[caiodallaqua]

DoD: Being able to call KeyExists for a key that is not in the DB without any logging errors.

The logged error points to Get, which makes it hard to debug. It should not log since checking errors is part of the logic here (due to Badger). Log example: level=ERROR msg="Key not found" trace=vectoria:src:internal:storage:Get