#3525338: Handle data fetching errors in PageInfo component

I initially thought the error (#3523029: Content entity form: URL alias without leading slash should be prevented by client-side validation) is being thrown while fetching the layout, so I started by wrapping that in more specific error boundaries and moving the component deeper inside the component tree. It turned out to be not the source of the problem, but I left that treatment in place, because it is useful.

Then discovered that the root cause was the PageInfo component, so I made the necessary changes there. I introduced a new method for reading the errors resulted by RTK Query, we can start using that gradually at other places if we're happy with it.