gui: New rest endpoint to get errors when web UI is opened

Since #4340 pulls aren't happening every 10s anymore and may be delayed up to 1h.
This means that no folder error event reaches the web UI for a long time, thus no
failed items will show up for a long time. Now errors are populated when the
web UI is opened.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4650
LGTM: AudriusButkevicius

lib/model/model_test.go

@@ -3229,7 +3229,7 @@ func TestPausedFolders(t *testing.T) {
 	}
 	w.Wait()
 
-	if err := m.ScanFolder("default"); err != errFolderPaused {
+	if err := m.ScanFolder("default"); err != ErrFolderPaused {
 		t.Errorf("Expected folder paused error, received: %v", err)
 	}