Fix PRESUBMIT errors ignored if there are warnings

R=gavinmak@google.com

Fixed: 1264947
Change-Id: If5163b167921f12cf91ca2efe3bbd1765109ba00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3360902
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

presubmit_support.py

@@ -1715,7 +1715,7 @@ def DoPresubmitChecks(change,
 
     if not should_prompt and not presubmits_failed:
       sys.stdout.write('%s presubmit checks passed.\n' % python_version)
-    elif should_prompt:
+    elif should_prompt and not presubmits_failed:
       sys.stdout.write('There were %s presubmit warnings. ' % python_version)
       if may_prompt:
         presubmits_failed = not prompt_should_continue(