See the docs of OError.tag:
https://github.com/overleaf/o-error#long-stack-traces-with-oerrortag
(currently at 221dd902e7bfa0ee92de1ea5a3cbf3152c3ceeb4)
I am tagging all errors at each async hop. Most of the controller code
will only ever see already tagged errors -- or new errors created in
our app code. They should have enough info that we do not need to tag
them again.