Want a way to "expect failure"

[gdamore]

This is really more for our own internal testing of C Convey... we'd really like to test assertion failures, and trap those failures without percolating the failed state up. What's called for here is some kind of "ExpectFailure()" or similar. This should actually invert the failure state of the current convey, so that a failure is considered "normal", although the Convey block still needs to cease execution.

This could actually include a code block, which executes in the failed context, allowing for further validation of the nature of the failure. This would greatly facilitate deep validation of our own failure handling modes.