* hydra_update_gc_roots.pl registers build outputs that should be kept

as GC roots and removes GC roots to build outputs that should no
  longer be kept.

src/Hydra/root/build.tt

@@ -241,6 +241,10 @@
 
 <h2>Build products</h2>
 
+[% IF !available %]
+  <p class="error">Note: this build is no longer available.</p>
+[% END %]
+
 [% PROCESS renderProductList %]
 
 [% END %]