Adjust lowlevel-tb hiding logic for `MsgStream`

Such that whenev the `self._ctx.chan._exc is trans_err` we suppress.
I.e. when the `Channel._exc: Exception|None` error **is the same as**
set by the `._rpc.process_messages()` loop (that is, set to the
underlying transport layer error), we suppress the lowlevel tb,
otherwise we deliver the full tb since likely something at the lowlevel
that we aren't detecting changed/signalled/is-relevant!

tractor/_streaming.py

@@ -595,8 +595,17 @@ class MsgStream(trio.abc.Channel):
             trio.ClosedResourceError,
             trio.BrokenResourceError,
             BrokenPipeError,
-        ) as trans_err:
-            if hide_tb:
+        ) as _trans_err:
+            trans_err = _trans_err
+            if (
+                hide_tb
+                and
+                self._ctx.chan._exc is trans_err
+                # ^XXX, IOW, only if the channel is marked errored
+                # for the same reason as whatever its underlying
+                # transport raised, do we keep the full low-level tb
+                # suppressed from the user.
+            ):
                 raise type(trans_err)(
                     *trans_err.args
                 ) from trans_err