Handle admin-menu error via action dispatch in data layer

[getdave]

Sub Issue of #45435

Currently we not handling any errors in the admin-menu data layer

wp-calypso/client/state/data-layer/wpcom/sites/admin-menu/index.js

Lines 24 to 26 in 040142a

	export const handleError = () => {
	return null;
	};

We need to to decide on the UX if the API encounters an error. How do we handle this? Fallback nav items? Show error message?

More specific tasks include:

• Create a new action which updates admin-menu state to record the ADMIN_MENU_REQUEST_ERROR state.
• Reformat adminMenu state to be combined reducers for ui and items so that we can have a flag for the current state of the fetch for menu items (eg: dispatched, success, failed).
• Dispatch the correct action in the data-layer error scenario and update state accordingly.

[getdave]

@Automattic/ajax Any ideas on scenarios where we might need to handle an error from the API in a user-facing manner. If not then we can close out this task.

[obenland]

Let's close it out for now. We can reopen when we do come across a scenario for it.