* Get rid of "positive failures" and separate log phases.

* Specifically log, for a build, whether the top-level derivation
  failed, or some dependency failed, or Nix itself barfed.  In the
  latter case log the error output from Nix.

src/HydraFrontend/root/static/css/hydra.css

@@ -153,6 +153,11 @@ td.buildfarmMainColumn {
     white-space: pre;
 }
 
+.error {
+    color: red;
+    font-weight: bold;
+}
+
 pre.buildlog {
     border: 1px solid black;
     padding: 0.3em;

src/HydraFrontend/root/static/css/logfile.css

@@ -75,7 +75,7 @@ em.storeref:hover span.popup {
     font-size: larger;
 }
 
-.error {
+.errorLine {
     color: #ff0000;
     font-weight: bold;
 }