DGS-1546 Map JsonMappingException to 400 response

[xiaoyali97]

fix #9
jira: https://confluentinc.atlassian.net/browse/DGS-1546

Test
Unit test added.

Example response

 % curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" \
    --data '{"schema": {}}' \
    http://localhost:8081/subjects/Kafka-key/versions
{"error_code":400,"message":"Cannot deserialize instance of `java.lang.String` out of START_OBJECT token\n at [Source: (org.glassfish.jersey.message.internal.ReaderInterceptorExecutor$UnCloseableInputStream); line: 1, column: 12] (through reference chain: io.confluent.kafka.schemaregistry.client.rest.entities.requests.RegisterSchemaRequest[\"schema\"])"}%

[rayokota]

Thanks @xiaoyali97 , LGTM!