message: Improve error message when feature flags aren't available
diff: </li>
  {%- else -%}
  <li class="pure-menu-item">
  - <span class="documented-info">Feature flags data are not available for this release.</span>
  + <span class="documented-info">Feature flags are not available for this release because it was built before features were collected by docs.rs.</span>
  </li>
  {%- endif -%}
  </ul>
  {%- elif features is iterable -%}
  <p data-id="empty-features">This release does not have any feature flags.</p>
  {%- else -%}
  - <p data-id="null-features">Feature flags data are not available for this release.</p>
  + <p data-id="null-features">Feature flags are not available for this release because it was built before features were collected by docs.rs.</p>
  {%- endif -%}
  </div>
  </div>
  