MINOR: http-htx: Add functions to read a raw error file and convert it in HTX

MINOR: http-htx: Add functions to read a raw error file and convert it in HTX

http_parse_errorfile() may now be used to parse a raw HTTP message from a
file. A status code is passed as parameter. It must be one of the supported HTTP
error codes to be valid. Otherwise an error is returned. It aims to be used to
parse "errorfile" directives. It relies on http_load_errorfile() to do most of
the job, ie reading the file content and converting it in HTX.