MEDIUM: proxy: remove http-errors limitation for dynamic backends

MEDIUM: proxy: remove http-errors limitation for dynamic backends

Use proxy_check_http_errors() on defaults proxy instances. This will
emit alert messages for errorfiles directives referencing a non-existing
http-errors section, or a warning if an explicitely listed status code
is not present in the target section.

This is a small behavior changes, as previouly this was only performed
for regular proxies. Thus, errorfile/errorfiles directives in an unused
defaults were never checked.

This may prevent startup of haproxy with a configuration file previously
considered as valid. However, this change is considered as necessary to
be able to use http-errors with dynamic backends. Any invalid defaults
will be detected on startup, rather than having to discover it at
runtime via "add backend" invokation.

Thus, any restriction on http-errors usage is now lifted for the
creation of dynamic backends.