If AJAX request fails - provide user-friendly way to see error

[romaninsh]

Sometimes AJAX request fails and it gives you this loading window forever:

I think JavaScript code should be able to pick up this problem and instead of only
showing it in the console, provide message like this:

"Server request failed. To see error click here". This can also be different (see implementation of form submission).

The link could point to the URL where request was sent.

This is mostly relevant for Modal windows. Form already have a pretty decent
error handling.