move-instance: Use error message instead of multiple state variables

Until now, move-instance used different status variables: “success”,
“abort” and “error_message”. With this patch, everything is changed
to use “error_message” only. This simplifies the code a bit.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>