No finished update with export mesh, and no 0 mesh size check

[andmccall]

I've been exporting some very large stacks as mesh and have had a couple issues with the Wavefront exporter. First, if the mesh list is of size 0, no error is produced (a 0 size mesh happened when I tried to use the max pixel value as the threshold value, I'm guessing a > is used somewhere instead of a >=). Second, there is no way to tell when the file has been fully exported as their is no dialog popup or showStatus update.