Fix the downloads to not stop at Interlude 3½

Apparently encoding the request URL is no longer a good idea, and generates a 404. Also, properly bail on 404s, instead of just counting it as the last chapter.

lib/download.js

@@ -124,10 +124,12 @@ function retry(times, fn) {
 }
 
 function downloadChapter(url) {
-  // Necessary for https://parahumans.wordpress.com/2011/10/11/interlude-3½-bonus/
-  const escapedUrl = encodeURI(url);
-
-  return retry(3, function () {
-    return request(escapedUrl).redirects(10);
+  return retry(3, () => {
+    return request(url).redirects(10).then(response => {
+      if (response.status !== 200) {
+        throw new Error(`Response status for ${url} was ${response.status}`);
+      }
+      return response;
+    });
   });
 }