Why is this build considered a failure by hydra?

[Mark H Weaver]

Looking at http://hydra.gnu.org/jobset/gnu/master , it seems that a
large percentage of our builds have started failing.  I was curious so I
took a look.  The first failure I looked at was 'file', since I recently
upgraded it:

  http://hydra.gnu.org/build/46637

Looking at the tail of the log, it looks like a successful build.
However, hydra reports that it failed with exit code 100.

What's going on here?

     Mark