Develop error handling framework

[sadovnik]

I need to develop some error handling framework for the client-side so users don't get confused every time something is broken in the backend.

Here are some quick thoughts on requirements:

1. Every request that browser makes could return an error thus every request should have an error handler;
2. There must be an option to specify a custom error message for each request and for each error case (e.g. 500 error, 503 error and so on);
3. Error messages must be helpful if possible. If there's no custom message, a general message must be shown.