Major bug: No error output in a callback function!

I am using jupyterhub 1.1.4 and I realised just now that there is no error output when you develop a function for a callback. Just take a look at this piece of code:

from ipywidgets import widgets
from IPython.display import display

def testfun(args):
    i = 'a'.index('b')
text = widgets.Button(description='Example Text')

It seems to work. Does it? No: There is an error. BUT YOU WILL NOT GET ANY INFORMATION THAT THEREW IS AN ERROR. There is no output at all!!

I think that needs to be fixed asap.

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