Monitor the update-downloads script

[carols10cents]

We've had multiple cases where the update-downloads script was erroring and no one caught it until someone mentioned their crate's download counts weren't updating days later.

Someone should get paged if this happens, which we could detect by:

• The update-downloads script aborting the transaction with an error
• The number of version_downloads records with processed = 'f' getting over some threshold

I'm not sure which would fit with our existing paging infrastructure best.