Add error handling

[Marina-Banov]

Instead of simply logging that an error has occurred, there should be some UI indicator to inform the user why the app isn't working. Also, add loaders for actions that take a long time.

EXAMPLE
File: hr.uniri.szsur.ui.home
Function: getEvents

    when (response) {
        is NetworkError -> {
            Log.i("getEvents", "NO CONNECTION")
            ArrayList()
        }
        is GenericError -> {
            Log.i("getEvents", "ERROR ${response.code}")
            ArrayList()
        }
        is Success -> response.value as ArrayList<Event>
    }