When evaluating the code interactively, there's currently no
notification about evaluation errors. Exceptions in the output buffer go
simply unnoticed, unless you watch for them closely. I like to keep the
output buffer buried, and assume that no-notification==no-error.

Exceptions caused by interactive evaluation should either:

- show a notification of sorts in the minibuffer
- display the output buffer for inspection

For inspiration about the second, see:

https://github.com/wavexx/python-x.el/blob/master/python-x.el#L424

Given previous experience with python-mode.el (which tends to break the
current window layout with his own splitting logic), I find it important
that the notification function (which displays the output buffer) should
be customizable, and this is reflected in the example code.