Graceful handling of failures to load latest rev

When trying to resolve an edit conflict, we sometimes fail to load the
latest revision (e.g. if the entity is missing from wb_entity_per_page).
This used to trigger a fatal error. This change turns that into
a logged warning, and continues without resolving the redirect.

Caveat: no unit test, since this only happens with a corrupt database.

Bug: 70347
Change-Id: Ice24df04a915a4f7d25a6f8da8fa2ce94e034401
Merge "Graceful handling of failures to load latest rev"