bitbake: cooker: mark setFeatures command as read-only

This patch makes sure that the setFeatures command is marked
as read-only and that it can only run if the cooker is in
the initial state.

Additionally, remove logging from the XMLRPC module in favor
of sending the exception to the client for easy processing.

	[YOCTO #6089]

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>