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/rofolder.go

@@ -66,3 +66,7 @@ func (f *sendOnlyFolder) Serve() {
 func (f *sendOnlyFolder) String() string {
 	return fmt.Sprintf("sendOnlyFolder/%s@%p", f.folderID, f)
 }
+
+func (f *sendOnlyFolder) PullErrors() []FileError {
+	return nil
+}