save function should exit via common code

[rjrodger]

3 times, so DRY needed: https://github.com/senecajs/seneca-mongo-store/blob/master/mongo-store.js#L173 https://github.com/senecajs/seneca-mongo-store/blob/master/mongo-store.js#L202 https://github.com/senecajs/seneca-mongo-store/blob/master/mongo-store.js#L228