These error codes can provide more actionable information to a client
about the failed state of their mix, instead of simply being
disconnected from the server for an unknown reason and the client
seeing an EOF.

Currently two codes are defined: one for a catch-all server abort of
the session for an unspecified reason, and a second indicating that
the unmixed data submitted by the client (in a coinjoin, the tx
containing inputs and change outputs) is invalid.