feat: add message about mirror when nvd is down

[AryanBakliwal]

fixes #3547

Added some code to catch exceptions when we can't connect to NVD, and fall back to the mirror.
Additionally, print a message rather than a traceback when mirror is also down.

[AryanBakliwal]

@terriko @mastersans PTAL and let me know if any changes are required.

$ cve-bin-tool -d CURL,EPSS,GAD,OSV,PURL2CPE,REDHAT,RSD,OSV -u now -n json-nvd

[12:48:29] INFO     cve_bin_tool - CVE Binary Tool v3.4                                                                            cli.py:630
           INFO     cve_bin_tool - This product uses the NVD API but is not endorsed or certified by the NVD.                      cli.py:631
           INFO     cve_bin_tool - For potentially faster NVD downloads, mirrors are available using -n json-mirror                cli.py:634
           INFO     cve_bin_tool - Not using an NVD API key. Your access may be rate limited by NVD.                               cli.py:655
           INFO     cve_bin_tool - Get an NVD API key here: https://nvd.nist.gov/developers/request-an-api-key                     cli.py:656
           WARNING  cve_bin_tool - Using legacy JSON interface.                                                                    cli.py:661
           WARNING  cve_bin_tool - You may want to switch to using cve-bin-tool's mirrors using -n json-mirror                     cli.py:662
           INFO     cve_bin_tool - Disabling data source CURL                                                                      cli.py:777
           INFO     cve_bin_tool - Disabling data source EPSS                                                                      cli.py:777
           INFO     cve_bin_tool - Disabling data source GAD                                                                       cli.py:777
           INFO     cve_bin_tool - Disabling data source OSV                                                                       cli.py:777
           INFO     cve_bin_tool - Disabling data source PURL2CPE                                                                  cli.py:777
           INFO     cve_bin_tool - Disabling data source REDHAT                                                                    cli.py:777
           INFO     cve_bin_tool - Disabling data source RSD                                                                       cli.py:777
           INFO     cve_bin_tool - Disabling data source OSV                                                                       cli.py:777
[12:48:30] WARNING  cve_bin_tool.CVEDB - Updating cachedir /home/adminuser/.cache/cve-bin-tool                                   cvedb.py:667
           INFO     cve_bin_tool - Getting NVD CVE data...                                                                  nvd_source.py:410
           INFO     cve_bin_tool.CVEDB - NVD appears to be down, falling back to mirror                                     nvd_source.py:429
           INFO     cve_bin_tool.CVEDB - Rolling back the cache to its previous state                                            cvedb.py:812
           INFO     cve_bin_tool - [Using NVD API 2.0]                                                                      nvd_source.py:461
           INFO     cve_bin_tool.CVEDB - Fetching metadata from NVD...                                                         nvd_api.py:165
[12:48:32] INFO     cve_bin_tool.CVEDB - There are 229695 CVE entries in the database                                            cvedb.py:391
           INFO     cve_bin_tool.CVEDB - There are 229695 CVE entries from NVD in the database                                   cvedb.py:393
           INFO     cve_bin_tool - CVE database contains CVEs from National Vulnerability Database (NVD), Open Source              cli.py:921
                    Vulnerability Database (OSV), Gitlab Advisory Database (GAD) and RedHat
           INFO     cve_bin_tool - CVE database last updated on 15 April 2025 at 12:48:32

[terriko]

Approving the tests to run since this looks like it'll do the trick.

[terriko]

Flagging as needing changes since there's a bunch of test failures and also some linter issues:

cve_bin_tool/data_sources/nvd_source.py:426:13: F841 local variable 'e' is assigned to but never used
cve_bin_tool/data_sources/nvd_source.py:435:9: E303 too many blank lines (2)