Notify user of failed requests

[tscheepers]

If a request fails due to an incorrect implementation at the server side the user should be notified in some way.

Now the user is presented with an empty screen if the /main request fails and with a unresponsive interface if another request fails.

When displaying a message that a request has failed the frontend could provide more information by checking if it got a default error json, like Laravel provides in debug mode:

{"error":{"type":"InvalidArgumentException","message":"Route [auja.pages.menu] not defined.","file":"\/Users\/Thijs\/Development\/laravel\/NedapHealthcare\/vendor\/laravel\/framework\/src\/Illuminate\/Routing\/UrlGenerator.php","line":231}}