fpspreadsheet: Fix reading/writing of cell error values in biff. Insert/delete unit tests with formulas are correct now. Fix crash in BIFFExplorer related to reading of formulas.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3582 8e941d3f-bd1b-0410-a28a-d453659cc2b4