core: Add missing CDA message to AppErrors

Adding missing CDA error message of export image to the AppErrors file,
so the client will present the message instead of the enum place
holder ACTION_TYPE_FAILED_DISK_IS_BEING_EXPORTED.

Bug-Url: https://bugzilla.redhat.com/1024887
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Change-Id: I2ca5165d007296778f8a80e07c2e088db82e287f