[HOTFIX] path configuration for ghostscript for pdf preview

https://github.com/sulu-cmf/sulu-standard/pull/298
fix #82
@chirimoya @wachterjohannes imagick itself can only handle PDFs with 1 site else it throws an error and the page is running into 500 error. So we check if ghostscript successfully converted the PDF to Image else an MediaException is thrown to show the default icon.