Catch timeout errors in upload.py and retry

Started as: https://codereview.chromium.org/11446068/

Catching urllib2.URLError
These sometimes occur when an intermediate proxy is overloaded, and is only an temporary issue.
We want to catch these exceptions and retry uploading.

[M-A, 2012-12-10 15:08:44]

https://codereview.appspot.com/6912052/diff/1/third_party/upload.py
File third_party/upload.py (right):

https://codereview.appspot.com/6912052/diff/1/third_party/upload.py#newcode424
third_party/upload.py:424: elif e.reason == '[Errno 110] Connection timed out':
I'm not sure the error code is portable.

So I'd prefer with
  elif 'Connection timed out' in e.reason:

but see my comment below.

https://codereview.appspot.com/6912052/diff/1/third_party/upload.py#newcode431
third_party/upload.py:431: raise e
"raise" is better than "raise e" since it re-raise the original exception.
That's what you did at line 423.

I don't think it's useful to have 3 condition blocks, you really only need one;

if 'Connection timed out' in e.reason and tries <= 3:
  # retry with backoff
  (...)
  continue
raise

that's it.