CF computed_standard_name message needs improvement

[daltonkell]

When checking the computed_standard_name of a variable, this message is ambiguous:

§4.3 Vertical Coordinate
* §4.3.3 The standard_name of `sigma` must map to the correct computed_standard_name, `{'height_above_reference_ellipsoid', 'height_above_mean_sea_level', 'height_above_geopotential_da
tum', 'altitude'}

In the code, this message appears when the attribute does not exist. It should only appear when the attribute is incorrect; otherwise, the message should explicitly indicate its absence.

compliance-checker/compliance_checker/cf/cf.py

Lines 4618 to 4622 in 380e16a

	correct_computed_std_name_ctx.assert_true(
	getattr(variable, 'computed_standard_name', None) in _comp_std_name,
	'§4.3.3 The standard_name of `{}` must map to the correct computed_standard_name, `{}`'.format(vname, _comp_std_name)
	)
	ret_val.append(correct_computed_std_name_ctx.to_result())