Improve NetCore's message when archive is missing.

[cbiffle]

So @hawkw noticed this behavior:

$ cargo run -q -- --ip barf%eth0 probe
humility probe failed: invalid Zip archive: Invalid zip header

which confused us both. This is different from the normal "archive not provided" message, and probe itself doesn't require an archive at all!

It turns out to be a sequence of things:

• It appears that if you don't provide an archive, Humility internally makes a fake one with an archive byte string of &[].
• NetCore itself has an undocumented assumption that the HubrisArchive is not in this class.
• NetCore requires this no matter whether the command uses the archive, or not.

So, by inserting an explicit check in NetCore instead of letting the zip crate fail, we get somewhat better feedback:

humility probe failed: archive is required for network use but was not provided

[cbiffle]

I feel like, at some point, the HubrisArchive type will want to be renamed (since it exists whether or not there's an archive), and .archive() will want to return Option since its contents can be missing. I haven't done that in this PR.