pdns_control and rec_control need to handle failure better

[rubenk]

The line based protocol between pdns and pdns_control, and between recursor and rec_control, has no way to return failure. It returns various different messages when stuff goes wrong, like Empty line, Unknown command or Wrong number of parameters. Returning the message plus an error code in some structured format would be better.

[Habbie]

:+1:

[pieterlexis]

3038