Hello all,
This is a rather important topic with a rather simple solution:

1. Add to your build an Execute Shell
   
2.  Set the command to something like:

   if [ -f "$file" ]
   then
       echo "$file found."
   else
       echo "$file not found."
       exit 1
   fi

The important part for the failure is exit 1

If the condition meets it (and for what we care they can always meet it), exit 1 will fail the job.

Remembers that anything that fails (even plugins/syntax errors) will make the job fail.
Don’t fail at failing.