Clean up some error handling in mod_ssl:

* modules/ssl/ssl_engine_io.c: Define new error codes in the APR status
  code range, MODSSL_ERROR_BAD_GATEWAY and MODSSL_ERROR_HTTP_ON_HTTPS;
  use these HTTP_* constants directly as apr_status_t values.
  (ssl_io_filter_error): Use new constants.
  (ssl_io_filter_handshake): Return purely an APR status value rather
  than a mixture of HTTP_* constants, OpenSSL error codes, and APR
  status values.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@645549 13f79535-47bb-0310-9956-ffa450edef68