Exit with error if no filename given to -e switch

Exit with error if no filename given to -e switch

Before, 'lyx -e pdf2' would give no error and would exit with 0. A use
case is if a user has in a bash script the following command:

lyx -e pdf2 "${mylxyfile}" || exit 1

where 'lyx' is mispelled as 'lxy' and thus yields an empty
string. If LyX does not exit with an error, the script continues where
the user probably intends for it to stop.