[OCS-gw] Handle failures on pubsub

[mpeterss]

OCS-gw currenly does not support errors on pubsub.

We should consider having gRPC as a backup channel in case there is an error detected on pubsub.

We should handle malformed messages so that we do not get stuck in a loop where the top message can not get parsed.

[vihangpatil]

There are 4 types of messages between OCSGW and Prime.

1. CCR Initiate (OCSGW --> Prime)
2. CCR Update (OCSGW --> Prime)
3. CCR Terminate (OCSGW --> Prime)
4. Activate (OCSGW <-- Prime)

These are the suggested transport for these messages:

Message	Transport
CCR Initiate and CCR Terminate	Non-streaming gRPC
CCR Update	Streaming gRPC with Pub/Sub as backup
Activate	Pub/Sub