Add HOMEBREW_FORCE_BREW_WRAPPER_HELP_MESSAGE

Allow wrapper providers to append a custom help message
to the HOMEBREW_FORCE_BREW_WRAPPER error output. If set,
the message is included at the end of the error. If unset,
no change in behaviour.

This follows the same pattern as HOMEBREW_FORBIDDEN_OWNER
and HOMEBREW_FORBIDDEN_OWNER_CONTACT.