[Mac]Exit with unique status in symupload when file already exists.

- This change should also be made for other platforms.
- This allows users to tell the difference between upload succeeding, failing, and being skipped because the file already exists on server.

Change-Id: I0b404da7aac29e0a16346bbd816ad1c815985bce
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2341373
Reviewed-by: Mark Mentovai <mark@chromium.org>