Aperture will not open the library on a drive that is not formatted MacOS X Extended (Journaled), at least the newest Aperture versions are checking the formatting of the drive.
In addition to that - Apple advises against using network volumes, see: Use locally mounted Mac OS X Extended volumes for your Aperture library,
There is a risk of transmission errors and database corruption.
Also, the access to a NAS will be a lot slower than on a directly connected drive. Aperture will be less responsive. It is preferable, to have the Aperture library on the fastest drive you have.
See this long thread in the "More Like this" to the right: Re: Nas (Synology DS413j) and Aperture
If you want your NAS primarily for backup, create a writable disk image on the NAS with the correct formatting, and store the library on that disk image.