If we're in the middle of a cert-manager release we'll have a git tag
created for that release, but won't have a GitHub release created yet.
That means that an attempt to download old CRDs for our versionchecker
test will fail for that version, with a 404 error.
An alternative approach would be to use the GitHub API to query for
existing non-draft releases - but that introduces a new point of failure
whereby we can easily hit a rate limit, or else introduces the need for
a GitHub API token.
The GitHub API also has the issue that it doesn't present every release
in one API call, which complicates fetching releases using curl and uses
even more rate-limit capacity.
The approach here is simple; we ignore tags for which the release 404s,
download manifests for which the release gives a 200, and bubble up any
other errors.
Signed-off-by: Ashley Davis <ashley.davis@jetstack.io>