- Placed all cases of "unexpected bytes" errors into the
`UnexpectedMessage` variant.
- Placed all cases of "unexpected EOF" errors into the
`IncompleteMessage` variant. Description is now generic about
"connection closed before message completed", instead of mentioning
"request" or "response.
- Added `Error::is_incomplete_message()` accessor to help checking for
unexpected closures.
- Renamed some variants to be clearer when viewing the `Debug` format.
- Collected all "user" errors into an internal `User` enum, to prevent
forgetting to update the `is_user()` method.
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