make bgp send correct notification code/subcode in all cases

[taspelund]

There are a handful of error conditions in the BGP codebase (particularly in message parsing) which are handled in a sane, yet non-compliant, manner.

For example, if we are parsing a BGP message that has a valid HEADER but an otherwise invalid type field, we do not send a Notification with Message Header Error / Connection not synchronized.
There are a handful of other cases I saw just from visual inspection of the BGP parsing code (even with the MP-BGP work which expands upon this) which need this same treatment (many in the header parsing).