Fix ODF error constant roundtrip issue. [part of #724568]

2014-02-17  Andreas J. Guelzow <aguelzow@pyrshep.ca>

	* openoffice-read.c (oo_cell_start): handle gnm:error-value
	* openoffice-write.c (odf_write_cell): write gnm:error-value
	if we convert an error constant to a formula