MEDIUM: http-htx/proxy: Use a global and centralized storage for HTTP error messages

MEDIUM: http-htx/proxy: Use a global and centralized storage for HTTP error messages

All custom HTTP errors are now stored in a global tree. Proxies use a references
on these messages. The key used for errorfile directives is the file name as
specified in the configuration. For errorloc directives, a key is created using
the redirect code and the url. This means that the same custom error message is
now stored only once. It may be used in several proxies or for several status
code, it is only parsed and stored once.