The subnet error just means you're putting all your eggs in one basket!
There's an RFC, if my memory serves, that asks for two different physical locations (or at least subnets) for the Name Servers under a Domain. It's not always needed because one location might have multi-homed bandwidth or other resilience available but generally it's not that good an idea keeping all your NSs in one place.
Another school of thought is that if you're just serving some services alongside your NSs (eg your Name Server is on the same machine as your Web Server) and that service is no longer available, due to an outage of some sort, then what's the point in having backup DNS ? It won't make much difference to visitors.
Mail Servers tend to also hold undelivered mail for so long these days that even backup MX are hardly needed in practice, although I'm an advocate of secondary MX.
As for the first error ... no idea :-)