Make the CheckTreeIsOpen presubmit check fail on IOError.

Review URL: http://codereview.chromium.org/9264044

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@120586 0039d316-1c4b-4281-b951-d872f2087c98

presubmit_canned_checks.py

@@ -458,8 +458,9 @@ def CheckTreeIsOpen(input_api, output_api,
         long_text = status + '\n' + url
         return [output_api.PresubmitError('The tree is closed.',
                                           long_text=long_text)]
-  except IOError:
-    pass
+  except IOError as e:
+    return [output_api.PresubmitError('Error fetching tree status.',
+                                      long_text=str(e))]
   return []