To resolve the recently added and failing
`test_remote_exc_relay::test_unmasked_remote_exc`: never allow
`trio.Cancelled` to mask an underlying user-code exception, ever.
Our first real-world (runtime internal) use case for the new
`.trionics.maybe_raise_from_masking_exc()` such that the failing
test now passes with an properly relayed remote RTE unmasking B)
Details,
- flip the `Context._scope_nursery` to the default strict-eg behaviour
and instead stack its outer scope with a `.trionics.collapse_eg()`.
- wrap the inner-most scope (after `msgops.maybe_limit_plds()`) with
a `maybe_raise_from_masking_exc()` to ensure user-code errors are
never masked by `trio.Cancelled`s.
Some err-reporting refinement,
- always capture any `scope_err` from the entire block for debug
purposes; report it in the `finally` block's log.
- always capture any suppressed `maybe_re`, output from
`ctx.maybe_raise()`, and `log.cancel()` report it.
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