fix(relay): say which key was refused

The validation returns above the `log.info` that records a send_keys call, so a
refused key left no trace whatsoever: the client got a generic "keys contain
disallowed values" and the relay log stayed silent. That is exactly the case
worth diagnosing, because it is what a client newer than its relay looks like --
the client believes it sent a key, the relay believes it said why it would not,
and nothing on the server records which key was involved.

Both the log line and the error now name the offending keys, and only those --
a rejected array is usually one unknown key among several valid ones, and
echoing the whole array back makes the reader find it. The message is capped at
four keys of 24 characters each, and the log at 120 via %.120r, so a client
sending a large or hostile array cannot flood either.

An empty or non-list `keys` gets its own log line, since it means something
different: not an unknown key but a client that sent the wrong shape.