Thanks for the feedback, everybody, gist updated:
  https://gist.github.com/gavinandresen/7079034

Categories are:

0x01-0x0fProtocol syntax errors0x10-0x1fProtocol semantic errors0x40-0x4fServer
policy rule
<https://gist.github.com/gavinandresen/7079034#rejection-codes-common-to-all-message-types>

RE: why not a varint:  because we're never ever going to run out of reject
codes.  Eight are defined right now, if we ever defined eight more I'd be
surprised.

RE: why not use HTTP codes directly: because we'd be fitting round pegs
into square holes.

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Gavin Andresen