multi-outputs: invalid callback return gives bad error message

[alexcjohnson]

make some non-JSON-serializable (but still wrapped in a list) return to a multi-output callback:

@app.callback(
    [Output('c', 'children'), Output('d', 'children')],
    [Input('a', 'children'), Input('b', 'children')]
)
def c(a, b):
    return [Output('c', 'children'), Output('d', 'children')]

The error handler itself barfs because it's not expecting a multi-output spec:

  File "/Users/alex/plotly/dash/dash/dash.py", line 854, in _raise_invalid
    property=output.component_property,
AttributeError: 'list' object has no attribute 'component_property'