Add KCAS_ERR_CORE_IN_ACTIVE_STATE error code.

New error code will allow to properly handle issues caused by wrong
usage of `remove inactive core` command.
It will also allow to print meaningful error messages.

Signed-off-by: Slawomir Jankowski <slawomir.jankowski@intel.com>

casadm/extended_err_msg.c

@@ -256,7 +256,11 @@ struct {
 		KCAS_ERR_CACHE_STOPPING,
 		"Cache being stopped"
 	},
+	{
 
+		KCAS_ERR_CORE_IN_ACTIVE_STATE,
+		"Core device is in active state"
+	},
 
 };