tests: Verify that the pull error was from interruption

While working on https://github.com/GNOME/ostree/pull/84 I wanted to
verify that nothing else was going wrong.

tests/test-pull-resume.sh

@@ -39,9 +39,10 @@ maxtries=`expr $maxtries \* 2`
 
 for ((i = 0; i < $maxtries; i=i+1))
 do
-if ${CMD_PREFIX} ostree --repo=repo pull origin main; then
+  if ${CMD_PREFIX} ostree --repo=repo pull origin main 2>err.log; then
-    break;
+    break
-fi
+  fi
+  assert_file_has_content err.log 'error:.*Download incomplete'
 done
 if ${CMD_PREFIX} ostree --repo=repo fsck; then
     echo "ok, pull succeeded!"