open-behind: propagate errors from ob_wake_cbk

If opening fd in background fails, then remember the error and
fail all further calls on the fd.

Use the newly introduced call_unwind_error() function from
call-stub cleanup to fail the future calls.

Change-Id: I3b09b7969c98d915abd56590a2777ce833b81813
BUG: 846240
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4521
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/5808