linking: return after aborting transition

Avoid calling return_error() on the same transition multiple times.

Fixes: 4b153ec553 ("link-target.lua: change into a async hook")
See #628

src/scripts/linking/link-target.lua

@@ -50,6 +50,7 @@ AsyncEventHook {
             si_flags.failed_count > 5 then
           transition:return_error ("tried to link on last rescan, not retrying "
               .. tostring (si_link))
+          return
         end
 
         if si_props ["item.factory.name"] == "si-audio-virtual" then