Improve bot_update data pushing logic.
Improve bot_update data pushing logic.

Allows call to farm out 'failure' logic with a callback. _gsutil method
knows how to parse for error codes in _gsutil output (though the command doesn't
have very uniform error output).

This now removes the previous *_old files before moving the current start/end
over top of them. Additionally, it will remove the spurious retries from the
rm / mv commands.

R=agable@chromium.org, szager@chromium.org, hinoka@chromium.org
BUG=

Review URL: https://codereview.chromium.org/299253002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@273019 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build