color: handle image description whose creation gracefully failed

This patch is for our CM&HDR protocol extension implementation.

When we gracefully fail to create an image description, we send the
'failed' event and the client can only destroy such image description.
But nothing guarantees that clients will do the right thing.

This fixes some issues for not well behaved clients and also documents
how we handle image description whose creation gracefully failed
internally.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>