rgw: handle errors properly during GET on Swift's DLO.

Backport: Jewel, Hammer
Fixes: http://tracker.ceph.com/issues/15812
Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>
(cherry picked from commit 5ef0202)

Conflicts:
	src/rgw/rgw_op.cc (hammer uses ret instead of op_ret; enclosing if
			   block in hammer does not check skip_manifest)

src/rgw/rgw_op.cc

@@ -942,6 +942,7 @@ void RGWGetObj::execute()
     ret = handle_user_manifest(attr_iter->second.c_str());
     if (ret < 0) {
       ldout(s->cct, 0) << "ERROR: failed to handle user manifest ret=" << ret << dendl;
+      goto done_err;
     }
     return;
   }