Fix condition that arises when the server is configured as a proxy gateway (aka reverse

Fix condition that arises when the server is configured as a proxy gateway (aka reverse
proxy) and multiple processes are serving the same backend file at the same time.
 Only one of the processes will actually succeed at saving the file, the others will
fail with the message "[error] (17)File exists: proxy: error linking cache file ...".
Changing the message level from APLOG_ERR to APLOG_INFO will keep this message
out of the logs w/o major overhaul of this code.

PR: 2746

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@97837 13f79535-47bb-0310-9956-ffa450edef68