Throw ThreatAsWithdrawException on missing mandatory attributes
https://tools.ietf.org/html/rfc7606#section-3:
d. If any of the well-known mandatory attributes are not present in
an UPDATE message, then "treat-as-withdraw" MUST be used.
For this to work we need to update BGPTreatAsWithdrawException,
because RFC4271 requires us to report the attribute type.
JIRA: BGPCEP-359
Change-Id: I3f1a1d56a3cf1e7e1b84fe2f7a5933af95129b91
Signed-off-by: Matej Perina <matej.perina@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>