Emit to stderr every time we process a new file.

Summary:
If we're processing a bunch of files on stdin, it will say:
   processing exercise -
a lot, but it's better than nothing: at least we know it's working.
And when you list files on the commandline, the output will actually
be useful!

Test Plan:
Ran 'make exercises' and observed proper output:
   ... packing exercise rotation_of_polygons.html
   ... packing exercise rounding_numbers.html
   ... packing exercise rounding_whole_numbers.html
   ... packing exercise same_side_exterior_angles.html
   ... packing exercise same_side_exterior_angles_2.html
   <etc>

Reviewers: alpert, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D2092