Handle connection errors in browser

[Gormartsen]

err should be properly provided on:

•  Failed to load resource: Could not connect to the server.`
   XMLHttpRequest cannot load http://localhost:3100/todo due to access control checks.
  

•   Origin http://mfw.local is not allowed by Access-Control-Allow-Origin.
    Failed to load resource: Origin http://mfw.local is not allowed by Access-Control-Allow-Origin.
    XMLHttpRequest cannot load http://localhost:3100/todo due to access control checks.