MINOR: http-htx: Store errorloc/errorfile messages in http replies

MINOR: http-htx: Store errorloc/errorfile messages in http replies

During configuration parsing, error messages resulting of parsing of errorloc
and errorfile directives are now also stored as an http reply. So, for now,
these messages are stored as a buffer and as an http reply. To be able to
release all these http replies when haproxy is stopped, a global list is
used. We must do that because the same http reply may be referenced several
times by different proxies if it is defined in a default section.