Send-side `MsgTypeError`s actually shouldn't have any "boxed" traceback
per say since they're raised in the transmitting actor's local task env
and we (normally) don't want the ascii decoration added around the
error's `._message: str`, that is not until the exc is `pack_error()`-ed
before transit. As such, the presentation of an embedded traceback (and
its ascii box) gets bypassed when only a `._message: str` is set (as we
now do for pld-spec failures in `_mk_msg_type_err()`).
Further this tweaks the `.pformat()` output to include the `._message`
part to look like `<RemoteActorError( <._message> ) ..` instead of
jamming it implicitly to the end of the embedded `.tb_str` (as was done
implicitly by `unpack_error()`) and also adds better handling for the
`with_type_header == False` case including forcing that case when we
detect that the currently handled exc is the RAE in `.pformat()`.
Toss in a lengthier doc-str explaining it all.
Surrounding/supporting changes,
- better `unpack_error()` message which just briefly reports the remote
task's error type.
- add public `.message: str` prop.
- always set a `._extra_msgdata: dict` since some MTE props rely on it.
- handle `.boxed_type == None` for `.boxed_type_str`.
- maybe pack any detected input or `exc.message` in `pack_error()`.
- comment cruft cleanup in `_mk_msg_type_err()`.
0e8c60ee4a