Corbicula anti virus: Fixing bugs

I noticed two bugs in Corbicula over the last week, I fixed both of 'em:

• The Documents bookmark was always displayed in the Scan Target ListView, whether or not the ~/Documents directory actually existed. Now a check is made to ensure ~/Documents exists before the bookmark is added to the list.
• Corbicula interprets the ~/.gtk-bookmarks file to add the user's bookmarks to the Scan Target ListView, the FileNotFoundException is now caught properly and an empty bookmarks list is returned when the ~/.gtk-bookmarks file is not found.