[cas] reset error value before processing cell

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@29300 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#3718 fix

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@29301 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e