404: also be more helpful about non-existing book pages

Just like was just done for non-existing documentation pages, let's be
more helpful for missing book pages, too.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

content/404.html

@@ -12,7 +12,7 @@ <h1>That page doesn't exist.</h1>
   </p>
   <script>
     /* Be more helpful for outdated manual versions */
-    const match = window.location.pathname.match(/^(.*\/docs\/([^/]*))\/([0-9.]*)$/)
+    let match = window.location.pathname.match(/^(.*\/docs\/([^/]*))\/([0-9.]*)$/)
     if (match) {
       const [, path, command, version] = match
 
@@ -57,5 +57,14 @@ <h1>That page doesn't exist.</h1>
         }">${versions[i]}</a>.`
       })().catch(console.error)
     }
+
+    match = window.location.pathname.match(/^(.*\/book\/([^/]*))(\/.*)$/)
+    if (match) {
+      const [, path, rest] = match
+
+      const el = document.querySelector('#explanation')
+      el.innerHTML = `This book page was not found. <a href="${
+        path}${window.location.search}${window.location.hash
+      }">The book's front page is here</a>.`
   </script>
 </div>