Make tree-closed a warning for presubmit --all

presubmit --all should succeed regardless of the current state of the
tree. Otherwise a presubmit --all bot will be flaky due to circumstances
beyond its control.

This is a logical follow-on to crrev.com/c/3628368 which turned some
other errors into warnings for presubmit --all.

Bug: 1309977
Change-Id: I6d26708f8c47916cf015dec75e27833db7ca465b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3696070
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

presubmit_canned_checks.py

@@ -712,6 +712,10 @@ def CheckTreeIsOpen(input_api, output_api,
       if not status['can_commit_freely']:
         short_text = 'Tree state is: ' + status['general_state']
         long_text = status['message'] + '\n' + json_url
+        if input_api.no_diffs:
+          return [
+              output_api.PresubmitPromptWarning(short_text, long_text=long_text)
+          ]
         return [output_api.PresubmitError(short_text, long_text=long_text)]
     else:
       # TODO(bradnelson): drop this once all users are gone.