Need to get the right string representation of error cells.

Based on the code it seems a regression from:
d4cd8677

Change-Id: I30001e63cd17ae68ba4988d5825de479a4496326
Reviewed-on: https://gerrit.libreoffice.org/43146Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>