Adding a second optional message in progress-reporter
provides more control in the output format.
When both, NEW-MESSAGE and NEW-MESAGE2
are provided to `progress-reporter-force-update', the
structure of the output string would be as follows:

1) MESSAGE: it may be the operation under course.
2) As before, the status index (i.e., 30%).
3) MESSAGE2: it may be, i.e., the current file being processed.

In following example, the status index changes the position with
every message, which is distracting.

emacs -Q:
(let* ((games '("Mega Man"
 			   "Street Fighter II: The World Warrior"
 			   "Double Dragon"
 			   "Very long game with lot of stages: requires many months to complete"
 			   "The Legend Of Zelda"
 			   "Pac-Man"))
 	   (num-games (length games))
 	   (text "Checking game...")
 	   (prep (make-progress-reporter text 0 num-games)))
   (dotimes (i num-games)
 	(progress-reporter-force-update prep i
 					(format "Checking game... (%s) " (nth i games)))

 	(sit-for (or (and (= i 3) 5) 1)))
   (message "Games check completed!"))

; A second optional string allows to present the information as described in 1-2-3 above:
(progress-reporter-force-update prep i text
 				(format "(%s)" (nth i games)))

; This would display the index string in the same position for each update.



In GNU Emacs 25.1.50.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.29)
  of 2016-03-10 built on calancha-pc
Repository revision: 780a605e1d2de4b975e6f1f29b491c9af419dcff