Log sentry error if see large blocks of contacts with same modified_on

[rowanseymour]

We're currently loading up to 500,000 contacts to index which is far from ideal, but was the only way to deal with hundreds of thousands of contacts having the same modified_on. We've gotten better at only ever updating contacts in small batches, so this could be lower.

If we load up to N contacts, and we get N contacts with the same modified_on, then we won't advance properly, so that's a sentry error and something that needs to be addressed ASAP.