net: bridge: netlink: add extack error messages when processing vlans

Add extack messages on vlan processing errors. We need to move the flags
missing check after the "last" check since we may have "last" set but
lack a range end flag in the next entry.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>