filter: basic: add error checking for close()

filter/basic.go

@@ -80,7 +80,10 @@ func NewBasic(dst io.WriteCloser, debug bool, t, p int) *Basic {
 // Implements io.Closer.
 func (bf *Basic) Close() error {
 	if bf.debug {
-		bf.file.Close()
+		err := bf.file.Close()
+		if err != nil {
+			return fmt.Errorf("file cannot be closed: %w", err)
+		}
 	}
 	return nil
 }