Provide more actionable error messages when a user attempts to utilise a tag in category which that tag isn't allowed in

Meta Feature
1

As an example, see:

https://discussion.fedoraproject.org/t/does-the-docs-infrastructure-support-any-graphing-syntax/163060/4

Discourse should recommend that I utilise the correct category, rather than merely inform me that the tag is disallowed.



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