BUG/MINOR: arg: fix error reporting for add-header/set-header sample fetch arguments

BUG/MINOR: arg: fix error reporting for add-header/set-header sample fetch arguments

The 'add-header %[samples]' parsing errors associated to http-request
and http-response are displayed with the wrong keyword.

Configuration entry:

   http-request set-header mon-header %[res.hdr(user-agent)]

Original error message:

   [WARNING] 323/150920 (16559) : parsing [haproxy.conf:36] : 'log-format' : sample fetch <res.hdr ...

After commit error message:

   [WARNING] 323/150929 (16580) : parsing [haproxy.conf:36] : 'http-request' : sample fetch <res.hdr ...