Get endpoint implementation

Please check exceptions to response codes mappings https://github.com/slovit/geochat-server/tree/develop/src/main/java/ca/cencol/geochat/mapper. They make it easier to return a particular response code. [Here's](https://github.com/slovit/geochat-server/blob/develop/src/main/java/ca/cencol/geochat/service/impl/RoomServiceImpl.java#L67) a usage example