Convert ErrorExit() calls back into an HTTPError exception so it can be properly handled.

Convert ErrorExit() calls back into an HTTPError exception so it can be properly handled.

Occasionally, AppEngine throws HTTP 500. In practice it's not a big deal but
upload.py converts them into sys.exit() call which is problematic. This breaks
the commit queue since this behaviors forces the CQ to quit.

R=dpranke@chromium.org
BUG=
TEST=


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=123775

[Dirk Pranke, 2012-02-27 19:02:28]

lgtm

[commit-bot: I haz the power, 2012-02-27 19:04:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/maruel@chromium.org/9480005/1

[commit-bot: I haz the power, 2012-02-27 19:11:16]

Change committed as 123775