Fix error reporting for bad Luxi arguments in QueryD

Currently, the query daemon would simply close the connection to the
client without issuing a response, if parsing the arguments failed;
the error was just logged.

Since this is very ugly from the client's point of view, we change it
so that the error is both logged and sent back to the
client. Furthermore, the execution block reporting is also cleaned up
a bit, to make the code more uniform.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>