Fix OS creation's error handling when pausing sync

Commit 41e1e79e introduced a feature in which when wait_for_sync is not
set, DRBD sync is paused during the OS installation.

Doing so, however, broke OS creation's error handling: the result value
from the instance_os_add RPC call was overwritten by the one of the
blockdev_pause_resume_sync call before there was a chance for it to
be raised and thus masking possible errors in the OS creation.

Note that the wipe method, from which the pause technique was inspired,
does not suffer from this bug.

Signed-off-by: Faidon Liambotis <faidon@noc.grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>