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/rwfolder.go

@@ -286,7 +286,7 @@ func (f *sendReceiveFolder) pull(prevIgnoreHash string) (curIgnoreHash string, s
 			// we're not making it. Probably there are write
 			// errors preventing us. Flag this with a warning and
 			// wait a bit longer before retrying.
-			if folderErrors := f.currentErrors(); len(folderErrors) > 0 {
+			if folderErrors := f.PullErrors(); len(folderErrors) > 0 {
 				events.Default.Log(events.FolderErrors, map[string]interface{}{
 					"folder": f.folderID,
 					"errors": folderErrors,
@@ -1797,11 +1797,11 @@ func (f *sendReceiveFolder) clearErrors() {
 	f.errorsMut.Unlock()
 }
 
-func (f *sendReceiveFolder) currentErrors() []fileError {
+func (f *sendReceiveFolder) PullErrors() []FileError {
 	f.errorsMut.Lock()
-	errors := make([]fileError, 0, len(f.errors))
+	errors := make([]FileError, 0, len(f.errors))
 	for path, err := range f.errors {
-		errors = append(errors, fileError{path, err})
+		errors = append(errors, FileError{path, err})
 	}
 	sort.Sort(fileErrorList(errors))
 	f.errorsMut.Unlock()
@@ -1880,13 +1880,13 @@ func (f *sendReceiveFolder) deleteDir(dir string, ignores *ignore.Matcher, scanC
 	return err
 }
 
-// A []fileError is sent as part of an event and will be JSON serialized.
-type fileError struct {
+// A []FileError is sent as part of an event and will be JSON serialized.
+type FileError struct {
 	Path string `json:"path"`
 	Err  string `json:"error"`
 }
 
-type fileErrorList []fileError
+type fileErrorList []FileError
 
 func (l fileErrorList) Len() int {
 	return len(l)