Add structured logging and better error handling

web/static/js/download-form.js

@@ -1,4 +1,4 @@
-import { api } from "./api.js";
+import { api, describeError } from "./api.js";
 
 const fields = {
   form: document.getElementById("download-form"),
@@ -61,7 +61,7 @@ async function submitDownloads(event, onCreated) {
       });
     } catch (error) {
       failedUrls.push(url);
-      if (!firstError) firstError = error.message;
+      if (!firstError) firstError = describeError(error);
     }
   }