[hma][good first issue] Tweak Message For Checkpointing Sanity Check Failure

[Dcallies]

https://github.com/facebook/ThreatExchange/blob/main/hasher-matcher-actioner/src/OpenMediaMatch/storage/postgres/impl.py#L344

If two fetch processes are running at the same time, one of them will hopefully fail this assert to try and avoid rewinding the checkpoint.

The message currently cryptically refers to this as a "race condition", but this actually means there are probably multiple instances of the fetch process.

Let's update the message to be less cryptic, and refer to multiple fetch processes or tasks.

[anaypurohit0907]

hi i would like to work on this issue, please guide me.