Improve error messages when a backend is cancelled by deadlock detection
Improve error messages when a backend is cancelled by deadlock detection

We send SIGINT to a backend that is cancelled due to a deadlock. That
approach ends up being a very confusing error message.

With this commit we intercept the error messages and show a more
meaningful error message to the user.

citusdata/citus,citusdata/citus,citusdata/citus,citusdata/citus,citusdata/citus