[BUG] when an unserializable object is nested within a valid object, PlotlyJSONEncoder throws misleading exceptions

[JonThom]

Describe your context

dash                 2.0.0
dash-core-components 2.0.0
dash-html-components 2.0.0
dash-table           5.0.0

Describe the bug

When a non json-serializable object is nested within a valid object, PlotlyJSONEncoder throws misleading exceptions.

Expected behavior
PlotlyJSONEncoder should throw an exception stating that the offending object is non-serializable.

Reproducible example
The following example reproduces two different misleading exceptions.

(NB: we allow Dash to call the callbacks and trigger the exceptions on initial render)

import dash
from dash import html
from dash.dependencies import Input, Output

app = dash.Dash(__name__)

app.layout = html.Div(
    [
        html.Button("output to 1", id="buttonA"),
        html.Button("output to 2 and 3", id="buttonB"),
        html.Div(id="output1"),
        html.Div(id="output2"),
        html.Div(id="output3")
    ]
)

@app.callback(
    [Output("output1", "children")],
    Input("buttonA", "n_clicks")
    )
def non_serializable_output_in_list(n_clicks):
    # nested not jsonifiable output inside a list
    out1 = {
        "nested_not_jsonifiable":{"a":1, "b":2}.values()
    }
    return [out1]

@app.callback(
    [
        Output("output2", "children"),
        Output("output3", "children")
    ],
    Input("buttonB", "n_clicks")
    )
def non_serializable_output_alongside_no_update(n_clicks):
    # nested non-jsonifiable object alongside a dash.no_update
    out2 = {
        "nested_not_jsonifiable":{"a":1, "b":2}.values()
    }
    out3 = dash.no_update
    return [out2, out3]

if __name__ == '__main__':
    app.run_server(debug=True)

[JonThom]

@alexcjohnson as described in comment to #1014