[fix bug 928936] Return invalid date string when date_mozillians does not validate.
[fix bug 928936] Return invalid date string when date_mozillians does not validate.

chirilo/mozillians,brian-yang/mozillians,mozilla/mozillians,johngian/mozillians,hoosteeno/mozillians,safwanrahman/mozillians,chirilo/mozillians,justinpotts/mozillians,akatsoulas/mozillians,ChristineLaMuse/mozillians,brian-yang/mozillians,akatsoulas/mozillians,fxa90id/mozillians,anistark/mozillians,fxa90id/mozillians,ChristineLaMuse/mozillians,brian-yang/mozillians,brian-yang/mozillians,akarki15/mozillians,justinpotts/mozillians,johngian/mozillians,mozilla/mozillians,johngian/mozillians,akatsoulas/mozillians,anistark/mozillians,johngian/mozillians,akarki15/mozillians,ChristineLaMuse/mozillians,safwanrahman/mozillians,chirilo/mozillians,fxa90id/mozillians,hoosteeno/mozillians,safwanrahman/mozillians,safwanrahman/mozillians,chirilo/mozillians,justinpotts/mozillians,fxa90id/mozillians,mozilla/mozillians,anistark/mozillians,justinpotts/mozillians,hoosteeno/mozillians,akarki15/mozillians,anistark/mozillians,mozilla/mozillians,akatsoulas/mozillians,hoosteeno/mozillians,akarki15/mozillians