lib/model: Remove incorrect/useless panics (#5881)

lib/model/folder_sendonly.go

@@ -68,7 +68,7 @@ func (f *sendOnlyFolder) pull() bool {
 		curFile, ok := f.fset.Get(protocol.LocalDeviceID, intf.FileName())
 		if !ok {
 			if intf.IsDeleted() {
-				panic("Should never get a deleted file as needed when we don't have it")
+				l.Debugln("Should never get a deleted file as needed when we don't have it")
 			}
 			return true
 		}

lib/model/model.go

@@ -248,10 +248,8 @@ func (m *model) StartFolder(folder string) {
 
 // Need to hold lock on m.fmut when calling this.
 func (m *model) startFolderLocked(cfg config.FolderConfiguration) {
-	if err := m.checkFolderRunningLocked(cfg.ID); err == errFolderMissing {
-		l.Warnln("Cannot start nonexistent folder", cfg.Description())
-		panic("cannot start nonexistent folder")
-	} else if err == nil {
+	_, ok := m.folderRunners[cfg.ID]
+	if ok {
 		l.Warnln("Cannot start already running folder", cfg.Description())
 		panic("cannot start already running folder")
 	}