Indexes with no objects in them should at least give a warning of some kind

If a bitarchive-machines is dead (which happens once in a while) and a user asks
for an index to be build out of metadata files on the dead machine the
index-server builds a very small but fully functional index without any warning
for the end user. He/she gets very confused becase it looks like the job(s) are
ready to browse but nothing is visible.
NOTE: This bug is originally from Bugzilla bug_id=1015.