Don't LOG.error on max_depth (by default).

Reverts part of a94b9b4 which added an extra LOG.error
statement when max_depth is hit.

This is causing spurious errors to get logged in some of our
projects which have adopted this changed.

Related to: https://bugs.launchpad.net/nova/+bug/1140133

Change-Id: Ie7939e41797da000dd8b269f905f351df0b7116d