lua/msgpack: fix crash while encoding error extension

Whether errors are encoded as a msgpack extension or not is determined
by the serializer_opts::error_marshaling_enabled flag. Although an
instance of serizlier_opts is passed to luamp_encode(), it doesn't
propagate it to luamp_encode_extension_box(). The latter encodes an
error as a msgpack extension if the error_marshaling_enabled flag is set
in serializer_opts of the current session. This leads to a bug when
luamp_encode() is called with error_marshaling_enabled unset while the
current session has the flag set:

 1. luaL_tofield() sets field->type to MP_EXT and field->ext_type to
    MP_UNKNOWN_EXTENSION, because the error_marshaling_enabled flag is
    unset:

    https://github.com/tarantool/tarantool/blob/b0431cf8f47e9d081f6a402bc18edb1d6ad49847/src/lua/serializer.c#L548

 2. Basing on the ext_type, luamp_encode_r() skips the MP_ERROR
    swtich-case branch for the default branch and calls the
    luamp_encode_extension callback:

    https://github.com/tarantool/tarantool/blob/b0431cf8f47e9d081f6a402bc18edb1d6ad49847/src/lua/msgpack.c#L203

 3. The callback implementation (luamp_encode_extension_box()) encodes
    the error, because the error_marshaling_enabled flag is set in the
    current session settings, and returns MP_EXT:

    https://github.com/tarantool/tarantool/blob/b0431cf8f47e9d081f6a402bc18edb1d6ad49847/src/box/lua/init.c#L420

 4. luamp_encode_r() assumes that the callback didn't encode the
    extension, because it returned MP_EXT, and encodes it again as
    a string:

    https://github.com/tarantool/tarantool/blob/b0431cf8f47e9d081f6a402bc18edb1d6ad49847/src/lua/msgpack.c#L209

This results in a broken msgpack content.

To fix this bug, let's do the following:
 - luaL_tofield() now sets ext_type to MP_ERROR unconditionally,
   irrespective of serializer_opts::error_marshaling_enabled.
 - luamp_encode_r() invokes the luamp_encode_extension callback for
   a MP_ERROR field only if error_marshaling_enabled is set. If the flag
   is unset, it proceeds with converting the field to string.
 - luamp_encode_extension_box() doesn't check serializer_opts anymore.
   It doesn't need to, because it's called iff error_marshaling_enabled
   is set.
 - YAML and JSON encoders are patched to handle the MP_ERROR field type
   by appending error::errmsg to the output (they use luaL_tofield()
   internally to determine the field type so they have to handle
   MP_ERROR).

This basically disables error encoding as msgpack extension
everywhere except returning an error from a Lua CALL/EVAL, in
particular:
 - when creating a tuple with box.tuple.new(),
 - when inserting an error into a space,
 - when encoding an error with the msgpack module.

This is okay, because the functionality has always been broken anyway.
We will introduce a separate msgpack encoder option to enable encoding
errors as MP_ERROR msgpack extension.

Looking at the code links above, one is likely to wonder why error
encoding was implemented via the encode extension callback in the first
place. The lua/msgpack module knows about the MP_ERROR extension and
even partially handles it so it'd be only natural to call the error
encoder function directly, as we do with decimals and uuids.
Unfortunately, we can't do it, because the error encoder is (surprise!)
a part of the box library. I filed a ticket to move it to the core lib,
see #6432.

Closes #6431