Prevent '500' error when admin uses private marker

A 500 error was being raised when an admin tried to use the
id of a private image owned by another user as a marker.

Do not return 500 in these cases.

Also added appropriate test coverage.

Fixes bug 1172719.

Change-Id: If77d45c684dd7d4dda9eefd519fc3d36fc57e07a
Merge "Prevent '500' error when admin uses private marker"