cve-update-nvd2-native: increase the delay between subsequent request failures

cve-update-nvd2-native: increase the delay between subsequent request failures

Sometimes NVD servers are unstable and return too many errors.
There is an option to have higher fetch attempts to increase the chances
of successfully fetching the CVE data.

Additionally, it also makes sense to progressively increase the delay
after a failed request to an already unstable or busy server.
The increase in delay is reset after every successful request and
the maximum delay is limited to 30 seconds.

Also, the logs are improved to give more clarity.

Signed-off-by: Dhairya Nagodra <dnagodra@cisco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>