Begin implementing a reporting mechanism

server/routes/exists.js

@@ -3,6 +3,9 @@ const storage = require('../storage');
 module.exports = async (req, res) => {
   try {
     const meta = await storage.metadata(req.params.id);
+    if (!meta || meta.dead) {
+      return res.sendStatus(404);
+    }
     res.set('WWW-Authenticate', `send-v1 ${meta.nonce}`);
     res.send({
       requiresPassword: meta.pwd