title: how to handle errors and status codes with server side events
body: <p>I am wondering about how to handle errors when sending data via SSE</p>
  <p><strong>Regular HTTP</strong></p>
  <p>If for example I want to fetch a <code>User</code> record by <code>id</code> via an http endpoint <code>/users/:id</code> and I encounter a non existing user I would respond with status code <code>404</code> and the client would handle it.</p>
  <p><strong>SSE</strong></p>
  <p>Now I'm wondering how this would look like for SSE. Let's say my endpoint is <code>/users</code> and takes a list of ids <code>?user_ids=[1, 2, 3]</code>. The endpoint should find each record and send the results one by one. If the user with id <code>3</code> cannot be found, what is the protocol to convey this to the client?</p>