Inform user if an output file name is missing on command line

[Patrick Storz]

I just tried to figure out for a while why the "--export-latex" command line option wasn't working. Eventually I realized I was missing the "--export-pdf=FILENAME" option that goes along with it.

While it obviously was my error it would be nice if Inkscape could warn the user in this case and explain what needs to be done to actually produce output.

The same is true for some other options (e.g. I did a quick test with "--export-area-drawing") which - if specified alone on the command line - will cause Inkscape to silently exit without any warning.