lib/model: Mark initial scan as finished even if failed and refactor (fixes #4103)

The mechanism to disallow manual scans before the initial scan completed
(#3996) , had the side effect, that if the initial scan failed, no further
scans are allowed. So this marks the initial scan as finished regardless of
whether it succeeded or not.

There was also redundant code in rofolder and a pointless check for folder
health in scanSubsIfHealthy (happens in internalScanFolderSubdirs as well).
This also moves logging from folder.go to ro/rw-folder.go to include the
information about whether it is send-only or send-receive

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4104

lib/model/folder.go

@@ -11,10 +11,10 @@ import "time"
 type folder struct {
 	stateTracker
 
-	scan                 folderScanner
-	model                *Model
-	stop                 chan struct{}
-	initialScanCompleted chan struct{}
+	scan                folderScanner
+	model               *Model
+	stop                chan struct{}
+	initialScanFinished chan struct{}
 }
 
 func (f *folder) IndexUpdated() {
@@ -25,7 +25,7 @@ func (f *folder) DelayScan(next time.Duration) {
 }
 
 func (f *folder) Scan(subdirs []string) error {
-	<-f.initialScanCompleted
+	<-f.initialScanFinished
 	return f.scan.Scan(subdirs)
 }
 func (f *folder) Stop() {
@@ -38,12 +38,7 @@ func (f *folder) Jobs() ([]string, []string) {
 
 func (f *folder) BringToFront(string) {}
 
-func (f *folder) scanSubdirsIfHealthy(subDirs []string) error {
-	if err := f.model.CheckFolderHealth(f.folderID); err != nil {
-		l.Infoln("Skipping folder", f.folderID, "scan due to folder error:", err)
-		return err
-	}
-	l.Debugln(f, "Scanning subdirectories")
+func (f *folder) scanSubdirs(subDirs []string) error {
 	if err := f.model.internalScanFolderSubdirs(f.folderID, subDirs); err != nil {
 		// Potentially sets the error twice, once in the scanner just
 		// by doing a check, and once here, if the error returned is