Handling of Exceptions from `coord(axis="x")`

[MoseleyS]

The IMPROVER code contains a few places where axes are selected using their abbreviation ("x", "y", "z", "t") rather than a full name. This is particularly useful if we don't know what the full name is.

The behaviour of Iris in the instance where the requested abbreviation does not match any coord is to return None, which will then raise an obscure error message when the code attempts to use the None object like a Coord object.

It would be helpful to developers and users of IMPROVER code if we trapped these cases and raised a useful error message. I believe we have not so far as we rely on the standardise plugin to ensure that our Cubes meet a specific standard.

Originally posted by @MoseleyS in #2017 (comment)