ICU syntax presubmit check: change to warning (was: error)

This CL changes any issues the ICU syntax presubmit check may find
from an error to a warning. The reason for this are false positives:
the current ICU syntax check flags some more rare ICU plural syntax
variants as invalid while they are actually correct (e.g. explicit
numeral statements beyond 1).
The long-term solution for this is to improve the ICU syntax check to
correctly recognize such syntax variants. For now, this CL prevents CQ
errors for CLs containing such syntax.

Bug: 1081730
Change-Id: I882a4e6f5ae11a0a5361782589b1705d652531b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270221Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782955}

PRESUBMIT.py

@@ -5174,7 +5174,7 @@ def _CheckStrings(input_api, output_api):
       'screenshots check.'))
 
   if icu_syntax_errors:
-    results.append(output_api.PresubmitError(
+    results.append(output_api.PresubmitPromptWarning(
       'ICU syntax errors were found in the following strings (problems or '
       'feedback? Contact rainhard@chromium.org):', items=icu_syntax_errors))