BGS requestCrawl should return HTTP 400 or 502 or similar instead of 401 when it can't connect?

[snarfed]

Very minor, obviously! But this cost me 5-10m of debugging my JWT just now, when it turned out the problem was actually that the BGS couldn't connect to my PDS, before I looked at the actual response body. 400 or 502 would have helped avoid that.

Full response body: {"error":"given host failed to respond to ping: failed to decode xrpc error message (status: 404): invalid character \'\\u003c\' looking for beginning of value"}'

Could maybe be folded into #33; up to you all. Thanks in advance!