When there's an error, the kernel is supposed to send both an error message on iopub, and an execute_reply with largely the same contents on the shell channel. This feels redundant, and it's easy to miss one out without causing enough problems to notice - issue takluyver/bash_kernel#44 is because bash_kernel was only sending the error data in an execute_reply message, and that had gone unnoticed until now.

When we get round to making the next major version of the message protocol, I'd like to simplify this to make it more obvious. I might remove the error data from execute_reply, so it just contains 'status': 'error', and the error message has to be used for the details of what went wrong.