Forward RPC errors from crawler (#9569)

The `keepHTTPResponseAlive` would cause errors to be 
returned with status OK.

- Add '32' as a filler byte until a response is ready
- '0' to indicate the response is ready to be consumed
- '1' to indicate response has an error which needs
to be returned to the caller

Clear out 'file not found' errors from dir walker, since it may be 
in a folder that has been deleted since it was scanned.

cmd/data-usage.go

@@ -277,7 +277,7 @@ func (f *folderScanner) scanQueuedLevels(ctx context.Context, folders []cachedFo
 			size, err := f.getSize(Item{Path: path.Join(f.root, entName), Typ: typ})
 
 			sleepDuration(time.Since(t), f.dataUsageCrawlMult)
-			if err == errSkipFile {
+			if err == errSkipFile || err == errFileNotFound {
 				return nil
 			}
 			logger.LogIf(ctx, err)