Failing to marshal a package should not result in hard failure during publishing

[Qinusty]

Failing to marshal a single package currently blocks the publishing of all subsequent packages, this is undesirable.

https://github.com/ossf/package-feeds/blob/ce53f3ea/cmd/scheduled-feed/main.go#L45-L50 is the relevant code chunk.

[tom--pollard]

Seems sensible that this shouldn't lead to a complete loss of the subsequent package data. This might be somewhere to add another event, and simply continue.

[tom--pollard]

Now that #123 has merged I'll look to update publishPackages() to handle errors on a per package basis, using errPub to align with poll()