Report errors on unexpected HTTP Status codes

These are not technically transport errors, and Go's net/http doesn't
report them as such.  However, non-sensible codes don't make much
sense in the multi-request setting of htcat (it makes no sense to
splice a 404's body into a stream, for example), so escalate such
unexpected statuses to an error.

Per report of Naaman Newbold <naaman@heroku.com>.