Reduce WARNING-level logs in update_prevalence.py to at most one a day

[apiology]

WARNING logs for update_prevalence.py should indicate that a significant problem may exist that does not justify failing the prevalence run. They should ideally be rare enough that they get someone's attention when they happen.

We should address issues and get to the point where the current WARNING-level logs don't appear as frequently so that we can channel them into a visible area and get peoples' attention when they happen rather than being "expected" and fading into the background.

Current logs:

WARNING: 19,237,682 people affected by No county-level case data
WARNING: 38,246,108 people affected by No tests last week
WARNING: 67,427,787 people affected by No state-level case data
WARNING: 482,929,660 people affected by No cases noted for a week - State level
WARNING: 1,149,570,572 people affected by No cases noted for a week - Country level
WARNING: 61,437,847 people affected by No cases noted for a week - County level
WARNING: 342,217,509 people affected by Endpoints say no new cases and max(values) says new cases
WARNING: 4,822,233 people affected by Invalid test positivity rate
WARNING: 125,384 people affected by Negative correction is suspect

We should work through the above logs and identify next steps to mitigate the problem identified, then refine the logs based on the mitigations so we continue to have a good picture of the impact.

Success here might look like two weeks going by with zero warnings in the logs.