Detect draft CSS specs server hiccups

The server used to serve draft CSS specs sometimes returns the contents of the
directory instead of the actual spec. The code happily considered that the
content received in such cases was an actual spec and updated the spec title
accordingly, e.g. to "Index of css-animations-2".

This update makes the code throw an error when this happens. This will make the
job fail with an error even though there is nothing that we can really do about
it, but that seems preferable to silently committing invalid titles to
`index.json`.

src/fetch-info.js

@@ -276,26 +276,32 @@ async function fetchInfoFromSpecs(specs, options) {
         };
       }
     }
-    // Extract first heading
-    const h1 = dom.window.document.querySelector("h1");
-    if (h1) {
-      return {
-        nightly: { url: url },
-        title: h1.textContent.replace(/\n/g, '').trim()
-      };
+
+    // Extract first heading when set
+    let title = dom.window.document.querySelector("h1");
+    if (!title) {
+      // Use the document's title if first heading could not be found
+      // (that typically happens in Respec specs)
+      title = dom.window.document.querySelector("title");
     }
 
-    // Use the document's title if first heading could not be found
-    // (that typically happens in Respec specs)
-    const title = dom.window.document.querySelector("title");
     if (title) {
+      title = title.textContent.replace(/\n/g, '').trim();
+
+      // The draft CSS specs server sometimes goes berserk and returns
+      // the contents of the directory instead of the actual spec. Let's
+      // throw an error when that happens so as not to create fake titles.
+      if (title.startsWith('Index of ')) {
+        throw new Error(`CSS server issue detected in ${url} for ${spec.shortname}`);
+      }
+
       return {
-        nightly: { url: url },
-        title: title.textContent.replace(/\n/g, '').trim()
+        nightly: { url },
+        title
       };
     }
 
-    throw `Could not find title in ${url} for spec "${spec.shortname}"`;
+    throw new Error(`Could not find title in ${url} for ${spec.shortname}`);
   }));
 
   const results = {};