fdo#79249 call formula compiler with error string

... instead of attempting to stringize a NaN coded error value.

Regression introduced with 30a20743ae17e6e02183a65603d38968253b3ffb

Change-Id: Ia7a8ca39938820ac75db169404446fa696c6ee1b
construct grouped ScFormulaCell with bDirty=true, fdo#79249 related

Noticed when loading
https://bugs.freedesktop.org/attachment.cgi?id=99844
with fdo#79249 fix where oox::xls::applyCellFormulas() groups the
consecutive =#N/A formulas. Only A1 result was displayed, other cells
were displayed empty.

Change-Id: I3a541f74d3d25e1515a1c6d47f02ec6a8e817c93