make sure unused media exist before deleting them

aqt/main.py

@@ -982,6 +982,7 @@ will be lost. Continue?"""))
         mdir = self.col.media.dir()
         for f in unused:
             path = os.path.join(mdir, f)
+            if os.path.exists(path):
                 send2trash(path)
         tooltip(_("Deleted."))
         diag.close()