Exceptions is a catch-all bucket for packets that are not expected. For example, you get a packet with vlan tags outside the allowed range, such as packets with an invalid ethernet header.
This is slightly different from drops, which can be caused by shaping or a system temporarily running out memory. You do not see many exception packets under normal circumstances. The exception packets are ultimately defined by each vSwitch implementation. Cisco Nexus 1000v defines its own stats. The only place VMware increments this stat is invalid ether type in the packet.
This is not used for the most part, but may be used in the future.