error: make iproto errors reuse mp_error module

After error objects marshaling was implemented in #4398, there
were essentially 2 versions of the marshaling - when an error is
sent inside response body, and when it is thrown and is encoded
in iproto fields IPROTO_ERROR and IPROTO_ERROR_STACK. That is not
really useful to have 2 implementation of the same feature. This
commit drops the old iproto error encoding (its IPROTO_ERROR_STACK
part), and makes it reuse the common error encoder.

Note, the encoder skips MP_EXT header. This is because

* The header is not needed - error is encoded as a value of
  IPROTO_ERROR_STACK key, so it is known this is an error. MP_EXT
  is needed only when type is unknown on decoding side in advance;

* Old clients may not expect MP_EXT in iproto fields. That is the
  case of netbox connector, at least.

Follow up #4398

@TarantoolBot document
Title: Stacked diagnostics binary protocol
Stacked diagnostics is described in details in
https://github.com/tarantool/doc/issues/1224. This commit
changes nothing except binary protocol. The old protocol should
not be documented anywhere.

`IPROTO_ERROR_STACK` is still 0x52, but format of its value is
different now. It looks exactly like `MP_ERROR` object, without
`MP_EXT` header.

```
IPROTO_ERROR_STACK: <MP_MAP> {
    MP_ERROR_STACK: <MP_ARRAY> [
        <MP_MAP> {
            ... <all the other fields of MP_ERROR> ...
        },
        ...
    ]
}
```

It is easy to see, that key `IPROTO_ERROR_STACK` is called
'stack', and `MP_ERROR_STACK` is also 'stack'. So it may be good
to rename the former key in the documentation. For example, the
old `IPROTO_ERROR` can be renamed to `IPROTO_ERROR_24` and
`IPROTO_ERROR_STACK` can be renamed to just `IPROTO_ERROR`.