@torabi12 posted a really interesting screenshot of what it looked like when NVD API2 failed while NVD was down:

          I can not update the DB using api2:

Originally posted by @torabi12 in #3541 (comment)

We should really make it easier for people to find out that if NVD is down they could fail over to the https://cveb.in mirror. I'd suggest if we have any sort of network failure with NVD that fails completely (e.g. do let it actually retry appropriately before failing over) we should print a big warning message explaining that NVD appears to be down and we're reverting to the mirror, then do that.

We should also update our documentation to make sure we explain how to switch to the mirror if you're having other weirdness with NVD (it happens sometimes, and is likely to happen more as they do some technical changes in December). The answer is you have to remove your NVD_API_KEY from your config/environment/command line flags, and I don't think that's terribly obvious.