[core] fix spacemacs/error-delegate

determine flycheck or emacs error backend using next-error-function instead of
next-error-find-buffer

core/core-funcs.el

@@ -340,8 +340,7 @@ current frame."
 Delegates to flycheck if it is enabled and the next-error buffer
 is not visible. Otherwise delegates to regular Emacs next-error."
   (if (and (bound-and-true-p flycheck-mode)
-           (let ((buf (ignore-errors (next-error-find-buffer))))
-             (not (and buf (get-buffer-window buf)))))
+           (not next-error-function))
       'flycheck
     'emacs))