Return 400 on invalid request body for PUT/POST apis, handle CosmosDB 429 error, add jacoco plugin

1. Currently our AddMember and CreateGroup APIs return 400 with no response body when given invalid input {"test":"test"}. Instead it should return { "code": 400, "reason": "Bad Request", "message": "name must not be null" }.

2. We want to return 429 to user when Cosmos DB gives us Request rate too large error so we have better analysis of our service. Originally we threw 500. This is an ExecutionException thrown when we execute gremlin query in ClusterGremlinConnector.

3. Add jacoco plugin.