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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validation_error_message(cls, spec, backends=None):
 """ Returns an options validation error message if there are any invalid keywords. Otherwise returns None. """

        try:
            cls.validate_spec(spec, backends=backends)
        except OptionError as e:
            return e.format_options_error()