message: crate_details: add tooltip to versions that failed to build
diff: {{#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"><i class="fa fa-fw fa-warning"></i> {{this.version}} (build failed)</a>
  + <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}}
  