For jupyter lab this should work (@Alasja)
from IPython.display import HTML
HTML('''<script>
var code_show_err = false;
var code_toggle_err = function() {
var stderrNodes = document.querySelectorAll('[data-mime-type="application/vnd.jupyter.stderr"]')
var stderr = Array.from(stderrNodes)
if (code_show_err){
stderr.forEach(ele => ele.style.display = 'block');
} else {
stderr.forEach(ele => ele.style.display = 'none');
}
code_show_err = !code_show_err
}
document.addEventListener('DOMContentLoaded', code_toggle_err);
</script>
To toggle on/off output_stderr, click <a onclick="javascript:code_toggle_err()">here</a>.''')