Stacking and merge queue - sporadic (maybe, permanent) failures when attempting to merge a multi-PR stack

[thehcma]

Here is an example of the notification email (the message itself is broken - hyperlink is missing - and not actionable ("unknown reason"):

[#404](https://github.com/the-hcma/repository-helpers/pull/404) was removed from the [merge queue]() due to an unknown reason.