orgit-topic-export: Wrap any errors as org-link-broken error

[Thaodan]

This enables handling of org-export-with-broken-links and gracefully
fail any error.

[tarsius]

You didn't say what error you got, but I assume forge-get-url was called on nil, because forge-get-topic was called with an id that doesn't exist in the local database anymore. My commit addresses that issue more cleanly and signals the error the same way that is being done in Org.

[Thaodan]

Yes that was the case, I assumed that catching any error during the export would be the goal not just that there is a topic id for which no topic exists anymore. Anyway good that's its fixed.