Capture error messages while stopping ipython clusters using batch (qdel, bkill) commands. Helps diagnosing issues during cluster shutdown, which are otherwise silently ignored.
Merge pull request #3600 from chapmanb/master

Improved logging for errors during BatchLauncher.stop

This is a logging/debugging improvement that catches and reports errors when shutting down clusters using a BatchLauncher (qdel/bkill). The previous behavior was to silently fail if the external command failed. The patch captures error output and reports potential issues to the log.