error with checkers when standard_name doesn't exists in CF name table (ash probability variable especially)

[matdeslandes]

Hello,

It seems that it's now allowed to use a standard_name that is not part of the table.

As stated in the convention : «The set of permissible standard names is contained in the standard name table».

[amcz]

We may not be able to satisfy the error checkers completely

[matdeslandes]

A standard name need to be defined, something like probability_of_mass_concentration_of_volcanic_ash_in_air_above_threshold

See https://github.com/orgs/cf-convention/discussions/459