plugins/netmask: Don't panic on invalid netmask

[purpleidea]

It looks like the error handling for this plugin is borked. I noticed
this while reading this code during my attempt to implement a DHCP
server. This also clarifies the error messages, since they were either
innaccurate, or duplicated which would make it impossible to tell which
code path the error actually came from.

NOTE: I didn't test this code, but it seems pretty trivial. I hope you have CI, but since this bug got through, then maybe not?

[Natolumin]

There's CI, but not everything has good test coverage, especially not error checks.

Please signoff your commit (read https://developercertificate.org/ then git commit --amend --signoff and force-push).
The test failure looks like an infra issue on travis's side, it should retry next time you push

Otherwise, looks fine, we can merge once that's done

[insomniacslk]

I've retried the failing CI job and it works just fine. @purpleidea can you update your commit with DCO as explained above? Thanks!

[insomniacslk]

Ping @purpleidea , still interested in merging this PR?

[skoef]

I guess this is superseded by #144

[purpleidea]

If you think it still needs fixing lmk and I can rebase. Otherwise close. Cheers