fix for loading sessions that _don't_ have an associated marks file (cf 09f4af8cfd)

have to use exists() to reference variable which may possibly not exist
-	(which makes sense, in retrospect)
also, don't have `vimS -l` list marks files as if they were session files
-	while I was in there, made the output from `vimS -lv` look much prettier
-	plus, `vimS -lv` now indicates which sessions have associated marks files