Create fake user when userinfo request fails

Quick and dirty fix for a bug that completely breaks the metadata panel.
Should be refactored later (for a ForeignDbRepo, a missing user is
normal, for anything else it should still be treated as an error
condition, although maybe not with a rejection).

Bug: 62019
Change-Id: I433ae2bd1334593d9c5bfe0272ce7207f3fdf723
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/267
(cherry picked from commit 822e129a37773d1452b642e9b6ca4efaabcf02c2)