change error reporting for yank errors

[syphar]

While digging into #1934 and the related sentry issue, I saw one thing:

after a build, we're already fetching the release-data from crates.io, and updating our release-record with the yank-status from the response.

docs.rs/src/docbuilder/rustwide_builder.rs

Lines 487 to 493 in 6b1505d

	let release_data = match self.index.api().get_release_data(name, version) {
	Ok(data) => data,
	Err(err) => {
	warn!("{:#?}", err);
	ReleaseData::default()
	}
	};

So this PR does:

• change a warn to error when the crates.io data can't be fetched.
• only log an error for the yank-state-update if we don't have a build queued, assuming the build will fix the yank-state

[syphar]

r? @Nemo157 @jyn514