Add more detailed message when no library targets found.

[jw013]

Resolves #723

[obi1kenobi]

Looks good! Might have to update a test or two though.

[jw013]

I updated the tests that check the output. I'm a little stumped about the remaining CI failures - I see mentions of rustdoc v29, though, so I assume that's unrelated?

[obi1kenobi]

Huh, a new rustdoc format version must have been released without me noticing... Curious 🤔

I'll fix that, thanks for bringing my attention to it!

[obi1kenobi]

Fixing the nightly build is blocked pending a new version of an upstream crate being published. But that's why it's a non-required step in our CI -- we can merge without it.

I've fixed the other step that was problematic, so this can merge.

Thanks again!