Error handling

[stephencelis]

@Shared persistence strategies can fail to load or save, but we have generally silently ignored such failures. This PR explores introducing loadError and saveError properties on the projected value, similar to SwiftData's @Query's fetchError.

@Shared(.fileStorage(.syncUps)) var syncUps: [SyncUp] = []

var body: some View {
  if let loadError = $syncUps.loadError {
    ContentUnavailableView {
      Label("Failed to load sync-ups", systemImage: "xmark.circle")
    } description: {
      Text(loadError.localizedDescription)
    }
  } else {
    ForEach(syncUps) { syncUp in
      // ...
    }
  }
}