MINOR: http_htx: split check/init of http_errors

MINOR: http_htx: split check/init of http_errors

Function proxy_check_errors() is used when configuration parsing is
over. This patch splits it in two newly named ones.

The first function is named proxy_check_http_errors(). It is responsible
to check for the validity of any "errorfiles" directive which could
reference non-existent http-errors section or code not defined in such
section. This function is now called via proxy_finalize().

The second function is named proxy_finalize_http_errors(). It converts
each conf_errors type used during parsing in a proper http_reply type
for runtime usage. This function is still called via post-proxy-check,
after proxy_finalize().

This patch does not bring any functional change. However, it will become
necessary to ensure http-errors can be used as expected with dynamic
backends.