Use more sensible behavior in presubmit checks.

presubmit_canned_checks.py

Index: presubmit_canned_checks.py
diff --git a/presubmit_canned_checks.py b/presubmit_canned_checks.py
index 09439fb8833e5dc2d70db3f1796a7fd7dd706bcd..ba8c3ed3dc8f2fe5c7a9fe735226ee9243777e9a 100755
--- a/presubmit_canned_checks.py
+++ b/presubmit_canned_checks.py
@@ -74,7 +74,7 @@ def CheckChangeHasNoTabs(input_api, output_api):
   """
   for f, line_num, line in input_api.RightHandSideLines():
     if '\t' in line:
-      return [output_api.PresubmitError(
+      return [output_api.PresubmitPromptWarning(
           "Found a tab character in %s, line %s" %
           (f.LocalPath(), line_num))]
   return []
@@ -103,14 +103,15 @@ def CheckLongLines(input_api, output_api, maxlen=80):
 def CheckTreeIsOpen(input_api, output_api, url, closed):
   """Checks that an url's content doesn't match a regexp that would mean that
   the tree is closed."""
+  assert(input_api.is_committing)
   try:
     connection = input_api.urllib2.urlopen(url)
     status = connection.read()
     connection.close()
     if input_api.re.match(closed, status):
       long_text = status + '\n' + url
-      return [output_api.PresubmitError("The tree is closed.",
-                                        long_text=long_text)]
+      return [output_api.PresubmitPromptWarning("The tree is closed.",
+                                                long_text=long_text)]
   except IOError:
     pass
   return []