Update release post item linter to provide a more clear error message

Why is this change being made?

The linter put in place to validate release post items validates categories against the data/categories.yml file. The error message seemed like it could be improved to allow the error to be more easily understood and resolved.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what