tests/functional: ignore errors when caching assets, except for 404

tests/functional: ignore errors when caching assets, except for 404

We see periodic errors caching assets due to a combination of transient
networking and server problems. With the previous patch to skip running
a test when it has missing assets, we can now treat most cache download
errors as non-fatal.

Only HTTP 404 is retained as fatal, since it is a strong indicator of
a fully broken test rather than a transient error.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20241217155953.3950506-32-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>