Improve error message when migration status fail

Commit 6a1434d7 (“Make migration RPC non-blocking”) changed the API
for reporting migration status, but has a small cosmetic bug: if the
migration status if failure, but the RPC itself to get the status
didn't fail, it shows the following error message:

  Could not migrate instance instance2: None

since it always uses result.fail_msg, irrespective of which part of
the if condition failed.

This patch simply updates the msg if not already set, leading to:

  Could not migrate instance instance2: hypervisor returned failure

Proper error display can be done once the migration status objects can
return failure information as well, beside status.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>