#71 Command failure protocol response

[Roman M. Parparov]

Currently a failed empire command (and even an unknown
empire command)
failure is reported as a regular data (C_DATA).

Solution I
failure line should be reported as some other protocol
number (let's
define C_FAIL?) and an unknown command might use C_BADCMD.
* This might break clients

Solution II
* Make C_PROMPT report the last command's status.