Handle Rekor shutdown properly

[haydentherapper]

From https://github.com/haydentherapper/rekor-log-monitor/actions/runs/7589738050/job/20675012429:

Monitoring certificate subject email
Monitoring subject email
Root hash consistency verified - Current Size: 60809297 Root Hash: ad17814f625fa2ea6daee2ba964431024049e759e342bf676d314e03d73eb2ca - Previous Size: 60793700 Root Hash edbd34489710c54b2c1e7c79f808594efd3af77bf925b06a5db88824750[13](https://github.com/haydentherapper/rekor-log-monitor/actions/runs/7589738050/job/20675012429#step:6:14)117
2024/01/19 22:20:40 error getting entries by index range: http2: server sent GOAWAY and closed the connection; LastStreamID=1, ErrCode=NO_ERROR, debug="server_shutting_down"
exit status 1
Error: Process completed with exit code 1.

We should backoff and retry.