Fix handle_error with custom error while exception is not dict data(#931)

[sleepingF0x]

flask-restful/flask_restful/__init__.py

Line 330 in a2cf3a3

data.update(custom_data)

maybe update fail when data is a list type, so I add type detect after getting e.data

        if not isinstance(data, dict):
            data = {'data': data}