A Matrix bot that allows you to subscribe to build failures for nixpkgs-update (aka r-ryantm).
Message the bot and type help to see a list of commands.
Limitations
The bot has no access to the actual exit code of the nixpkgs-update runners, so it simply looks for the presence of the word error in the logs.
This is obviously fallible, and can lead to false positives (e.g. here) and false negatives (feel free to report them).