(bug T83) If the Perso class is instancied with an non existent perso_id, application dies.

Summary:
This fix the following bug:
A perso externally deleted but still logged in can still use the site under the old perso_id.

Test Plan: Tested reproducing the scenario in T83.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T83

Differential Revision: http://code.zed.dereckson.be/D6