How to change logging format in Bokeh server

I’ve found that debugging is difficult in the Bokeh server, with errors that don’t generate a traceback like this one:

2016-09-18 13:54:52,871 error handling message Message 'PATCH-DOC' (revision 1): KeyError('the label [1.85642802177] is not in the [index]',)

So, I'd like to get more information, by including the line number in the message. I tried this:

handler = logging.StreamHandler()
handler.formatter = logging.Formatter(
‘[%(asctime)s] p%(process)s {%(pathname)s:%(lineno)d} %(levelname)s - %(message)s’,
‘%m-%d %H:%M:%S’,
root_logger = logging.getLogger()
root_logger.handlers = [handler]

But nothing changed. I realize this is likely to give the line number in Bokeh rather than my code where the error occurred, so maybe there is a better way. But as a first pass, can anyone help me set the handler on the root logger?