JSON serializable errors are hard to debug

[chriddyp]

If the json serializer fails, the traceback doesn't indicate which object couldn't be serialized or where to find it. For large apps, this becomes really difficult to track down. For example:

Traceback (most recent call last):
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
    response = self.handle_exception(e)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/dash/dash.py", line 556, in dispatch
    return self.callback_map[target_id]['callback'](*args)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/dash/dash.py", line 524, in add_context
    cls=plotly.utils.PlotlyJSONEncoder),
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/plotly/utils.py", line 136, in encode
    encoded_o = super(PlotlyJSONEncoder, self).encode(o)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/Users/chriddyp/Repos/dash-stuff/my-project/venv/lib/python3.6/site-packages/plotly/utils.py", line 204, in default
    return _json.JSONEncoder.default(self, obj)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type 'function' is not JSON serializable

[rmarren1]

Fixed in #273