message: navigation_rustdoc: show a warning next to releases that failed to build
  This introduces the same change as
  to navigation_rustoc.
diff: <div class="pure-menu pure-menu-scrollable sub-menu">
  <ul class="pure-menu-list">
  {{#each releases}}
  - <li class="pure-menu-item"><a href="/crate/{{../name}}/{{this.version}}" class="pure-menu-link">{{this.version}}</a></li>
  + <li class="pure-menu-item">
  + {{#if this.build_status}}
  + <a href="/crate/{{../name}}/{{this.version}}" class="pure-menu-link">{{this.version}}</a>
  + {{else}}
  + <a href="/crate/{{../name}}/{{this.version}}" class="pure-menu-link warn" title="docs.rs failed to build {{../name}}-{{this.version}}"><i class="fa fa-fw fa-warning"></i> {{this.version}}</a>
  + {{/if}}
  + </li>
  {{/each}}
  </ul>
  </div>
  