Don't set `Context._error` to expected `ContextCancelled`

If the one side of an inter-actor context cancels the other then that
side should always expect back a `ContextCancelled` message. However we
should not set this error in this case (where the cancel request was
sent and a `ContextCancelled` msg was received back) since it may
override some other error that caused the cancellation request to be
sent out in the first place. As an example when a context opens another
context to a peer and some error happens which causes the second peer
context to be cancelled but we want to propagate the original error.

Fixes the issue found in https://github.com/pikers/piker/issues/244

tractor/_streaming.py

@@ -425,7 +425,17 @@ class Context:
                 f'Remote context error for {self.chan.uid}:{self.cid}:\n'
                 f'{msg["error"]["tb_str"]}'
             )
-            self._error = unpack_error(msg, self.chan)
+            error = unpack_error(msg, self.chan)
+            if (
+                isinstance(error, ContextCancelled) and
+                self._cancel_called
+            ):
+                # this is an expected cancel request response message
+                # and we don't need to raise it in scope since it will
+                # potentially override a real error
+                return
+
+            self._error = error
 
             # TODO: tempted to **not** do this by-reraising in a
             # nursery and instead cancel a surrounding scope, detect