MINOR/OPTIM: http_htx: lookup once http_errors section on check/init

MINOR/OPTIM: http_htx: lookup once http_errors section on check/init

The previous patch has splitted the original proxy_check_errors()
function in two, so that check and init steps are performed separately.
However, this renders the code inefficient for "errorfiles" directive as
tree lookup on http-errors section is performed twice.

Optimize this by adding a reference to the section in conf_errors
structure. This is resolved during proxy_check_http_errors() and
proxy_finalize_http_errors() can reuse it.

No need to backport.