Currently, running rebase-update may lead to missing untracked files.
This change prints an error instead freezing and thawing local changes.

Bug: 1251047
Change-Id: Ia5f718ed957dae936d2ead2778760ae0c8d9f9ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3171828
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>