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

cmd/syncthing/mocked_model_test.go

@@ -132,3 +132,7 @@ func (m *mockedModel) State(folder string) (string, time.Time, error) {
 func (m *mockedModel) UsageReportingStats(version int, preview bool) map[string]interface{} {
 	return nil
 }
+
+func (m *mockedModel) PullErrors(folder string) ([]model.FileError, error) {
+	return nil, nil
+}