Use new `RemoteActorError` fields in various assertions particularly
ensuring that an RTE relayed through the spawner from the little_bro
shows up at the client with the right number of entries in the
`.relay_path` and that the error is raised in the client as desired in
the original use case from `modden`'s remote spawn spawn request API
(which was kinda the whole original motivation to finally get all this
multi-actor error relay stuff workin).
Case extensions:
- RTE relayed from little_bro through spawner to client when
`raise_sub_spawn_error_after` is set; in this case test should raise
the relayed and RAE boxed RTE right up to the `trio.run()`.
-> ensure the `rae.src_uid`, `.relay_uid` are set correctly.
-> ensure ctx cancels are no acked.
- use `expect_ctxc()` around root's `tell_little_bro()` usage.
- do `debug_mode` assertions when enabled by test harness in each actor
layer.
- obvi use new `.src_type`/`.boxed_type` for final error propagation
assertions.
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