core: Adjust VM Failure message(#907900)

When creation of a snapshot fails due to VM state, a more informative
message is delivered.
ACTION_TYPE_FAILED_VM_IS_SAVING_RESTORING used to be:
Cannot ${action} ${type}. VM is in saving/restoring state.\n\
	-Please wait until the VM's status has changed.

and is now:
Cannot ${action} ${type}. VM is in saving/restoring state.\n\
	-Please try again when the VM is either up or down.

Change-Id: I16a5c0437aefea9a6958741fbdcf84f989d8b61e
Bug-Url: https://bugzilla.redhat.com/907900
Signed-off-by: Vered Volansky <vvolansk@redhat.com>