transform WP_Error into the Mastodon API format

[pfefferle]

See https://docs.joinmastodon.org/entities/Error/

[akirk]

I am having some second thoughts here, I think we also need to ensure that we are modifying this error response only for a request that was handled by the EMA plugin. Maybe do use the third parameter WP_REST_Request $request and simply check if the rest route starts with api/? What do you say?

[pfefferle]

oh, good point!