tdf#112884: Error string is displayed as empty item in pivot table

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>

sc/source/core/data/dpcache.cxx

@@ -155,7 +155,7 @@ void initFromCell(
 
     if (rCell.hasError())
     {
-        rData.SetErrorStringInterned(internString(rStrPool, aDocStr));
+        rData.SetErrorStringInterned(internString(rStrPool, pDoc->GetString(rPos.Col(), rPos.Row(), rPos.Tab())));
     }
     else if (rCell.hasNumeric())
     {