```
result = Promise.all([<Promise that rejects eventually>]) # rejection 1
result.then () -> RoomEvents.emit(eventName) # rejection 2
result.catch (err) -> RoomEvents.emit(eventName, err) # handle r1
```
As shown above, the second rejection remains unhandled. The fix is to
chain the `.catch()` onto the `.then()` Promise.
This can now happen all the time, as we skip the join for clients that
disconnect before joinProject/joinDoc completed.
(cherry-picked from commit f357931de74e088800f3cced3898cce4f251dad0)