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>

modules/include/cas_ioctl_codes.h

@@ -589,6 +589,9 @@ enum kcas_error {
 
 	/** Cache already being stopped*/
 	KCAS_ERR_CACHE_STOPPING,
+
+	/** Core device is in active state */
+	KCAS_ERR_CORE_IN_ACTIVE_STATE
 };
 
 #endif