mod_proxy_balancer: fix error message implying recovery

during a markdown.

Backport of r1299738 from trunk resp. r1301194 from 2.4.x.

Submitted by: covener
Reviewed by: trawick, wrowe
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1374534 13f79535-47bb-0310-9956-ffa450edef68