Merge r1311172, r1331243 from trunk:

If there is an error reading the headers from the backend,
send an error to the client


add PR reference

Submitted by: sf
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1331416 13f79535-47bb-0310-9956-ffa450edef68