Improve error message when checkstyle cannot parse a .java file

Also adds a main() to checkstyle.py to make it easy to run it without
having to go through "git cl presubmit".

Bug: None
Change-Id: I462335b9b5e8f1f13e9a090c5d74d8f52449806e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4943930
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210269}