upload.py: Fix immutable exception attribute.
upload.py: Fix immutable exception attribute.

BUG=chromium:356813
TEST=local
  - Actually failed login, watched error message get printed.

Review URL: https://codereview.chromium.org/1063143003

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@294705 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools