Warn or error rather than returning None from name check in netcdf

[wjbenfold]

📰 Custom Issue

Based on my understanding of situations where the following code can end up returning None, I think that it's unlikely that the user doesn't want to know that this is happening.

https://github.com/SciTools/iris/blob/main/lib/iris/fileformats/netcdf.py#L344-L362

From conversation with @pp-mo, we think this means we're not saving valid CF because the aux_factory gets saved with a None formula term. That seems worth telling the user about.

[pp-mo]

IIRC I did introduce this 'return None if not present' intentionally, because it made some code much simpler
-- e.g. https://github.com/SciTools/iris/blob/main/lib/iris/fileformats/netcdf.py#L1531-L1537
.. which previously looked like this :

iris/lib/iris/fileformats/netcdf.py

Lines 1385 to 1391 in cc574cb

	if element not in self._name_coord_map.coords:
	cf_name = self._create_generic_cf_array_var(
	cube, dimension_names, element
	)
	self._name_coord_map.append(cf_name, element)
	else:
	cf_name = self._name_coord_map.name(element)

It changed in #4318

Unfortunately, I think I must have missed the fact that in some other usages, we purposely wanted this routine to fail in an invalid case.

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