REST: Respond an error if the incoming JSON can not be parsed

Someone tries to create a group via the REST API

PUT http://localhost:8080/olat/restapi/groups
{"description":"Test","externalId":'null',"managedFlags":'null',"name":"TestGroup","minParticipants":'null',"maxParticipants":'null'}

This request results in a https status 200 but the group is not created in OpenOLAT.

The reason for the missing group is that the parsing of the JSON fails because the 'null' (with apostrophe) can not be mapped to an Integer. In this case the http status 500 (Internal Server Error) should be responded instead of the http status code 200 (OK).