Move duplicate check before voiced-queue-space check

[JustAnotherArchivist]

If there is a queue of 5 or more jobs and a voiced user tries to !a a URL for which a job exists already, the bot responds with (#337):

Sorry, all pipelines are currently full, and only opped users can add to the queue beyond 5 pending. Please try again later.

This error is inaccurate in the sense that such a command wouldn't add to the queue. It should display "Job for X already exists." plus the status of the existing job instead, as it would for an opped user or if there was space in the queue.