mod_proxy: follow up to r1645529: 502 in case of SSL handshake failure.

Make the SSL filters chain return an error when the handshake fails with an
origin server. It can then be caught by mod_proxy to fail with 502.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1839442 13f79535-47bb-0310-9956-ffa450edef68