[FfmpegAdapter] Encode that fails inspection in #create stays in :running state and is uncancelable

[cjcolvar]

If an encode is created with an invalid file (e.g. image file) the mediainfo inspection fails and the encode object returned from #create has a status of :failed but subsequent reads via #find return an encode object with status :running. The encode does not have a pid file or an exit status code file so it always returns with status :running and running cancel! fails (it causes the rails console to bomb). In Avalon this leaves the polling job re-enqueuing eternally.

Related issues
#129
#130