Show an error message when the user tries to set the commit flag or add trybots to a private issue.

gcl.py

Index: gcl.py
diff --git a/gcl.py b/gcl.py
index 1502c061effe99e4b16cf2c2a0de0bd384f1ad4b..f6bb85286f5fc131fab9e97b046425e12389a417 100755
--- a/gcl.py
+++ b/gcl.py
@@ -1367,6 +1367,12 @@ def CMDtry(args):
   else:
     change_info = ChangeInfo.Load(args[0], GetRepositoryRoot(),
                                   True, True)
+
+  props = change_info.RpcServer().get_issue_properties(
+    change_info.issue, False)
+  if props.get('private'):
+    ErrorExit('Cannot use trybots on a private issue')
+
   if change_info.GetFiles():
     args = args[1:]
   else: