Expected ctx cancelled should not override a source error

[goodboy]

Fix for https://github.com/pikers/piker/issues/244 which addresses an issue where if a Context is cancelled due to one side erroring, we don't want to mask that orignal source error with the ContextCancelled which is responded back from the callee side which was cancelled as part of the caller side's error handling.

So in other words,

• caller opens a context with callee
• caller errors inside its context and the error handling sends a Context.cancel() request to the callee
• the callee side handles that cancel request and sends back a ContextCancelled
• the caller receives the ContextCancelled but should not override the local Context._error value with the cancel request response
• the caller should raise the original source error