bgpd: improve logging of invalid BGP Notifications

bgpd: improve logging of invalid BGP Notifications

Invalid BGP Notification messages should be logged locally, cf.
RFC4271, Sect. 6.4, p 34,
  NOTIFICATION Message Error Handling

Current notification for invalid Notification code:

  2012/10/10 02:17:54 BGP: message index 10 not found in bgp_notify_msg (max is 8)
  2012/10/10 02:17:54 BGP: 192.168.1.1 received NOTIFICATION 10/0 ((no item found)) 0 bytes

the logging should be a bit more clear. The above logging really doesn't
explain much and looks more like a programming error.

[rewrote most of it to get in something I can call a shape -David]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>