This is to make the script work even when config file is broken.

Change-Id: I6c2a090b01026b5747137bcc9fdec0bd5c03f75e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3607373
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>