Create a general error handler which handles multiple errors - Nia 1435

[tjmtic]

Fixes #1435

Data layer General Message Handler Interface and Implementation, with Sealed Type Definitions. Messages are extensible to both simple error messages and those with success/failure callbacks, so it can be used to handle both #1068 as well as any uses of snackbarHostState.showSnackbar(), such as for the Bookmark Saved Notification.

Multiple message sources combined in NiaAppState, which will be a single source of truth for user bound messages via the Snackbar per this conversation

Display values defined by UI via getSnackbarValues helper in NiaApp

Fake ErrorMonitor implemented as EmptyErrorMonitor to remove conflicting with other tests.