Better reporting for Mastodon API methods not implemented by a Node

[jernst]

E.g WordPress Enable Mastodon Apps plugin does not implement:

• self.http_get(f'/api/v1/statuses/{ obj_id }/favourited_by')
• self.http_get(f'/api/v1/accounts/{ this_account_id }/following')

In this case, we currently raise in the

requests.exceptions.HTTPError: HTTP status 404: {"error":"rest_no_route","error_description":"No route was found matching the URL and request method."}

This should be intercepted and reported as NotImplementedByNodeException.