[NNTP] [2504] Response code for "need MODE READER first"

[Clive D.W. Feather]

Russ Allbery said:
> The disadvantage of sticking with 502 for the MODE READER case is that
> then 401 isn't actually used by anything that currently exists, which
> always makes me a bit nervous.  But 502 is existing practice.
> 
> Leave 401 in, and I don't care a lot about 502 vs. 401 for MODE READER but
> I'd tend to lean towards 502 (which also lets us simplify the 401
> description slightly to always use an extension label).

I think it's finely balanced, but I'm going to go for 502 on the basis of:
- existing practice
- a transit-only server is going to be returning 502 for reader commands
  (and vice versa)
- CAPABILITIES is the right way to decide what the problem is (when we
  started we didn't have it).

I'm going to change the 401 wording to allow any capability name, rather
than only an extension name. That might come in useful one day.

-- 
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