Missing description of some functions and fields of error_object  

**Product:** Tarantool
**Since:** TBC
**Root document:** https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_error/error_object/
**SME:** @ Gerold103 , @ sergepetrenko  

# Details

* Describe the following function and fields of the `error_object`

  * `unpack()`
  * `raise()`
  * `code`
  * `type` 
  * `message`
  * `trace`
