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.
...and fix the stubbing of `io.sockets.clients`.
We were running the assertions prior to the actual completion of the
leaveProject. Which in turn shadowed the broken test setup --
`io.sockets.clients()` was returning the `this.clientsInRoom=[]` from a
previous context.
