Print failure reason when git cl try can't trigger jobs on a CL.

This is a very little fix.

Review-Url: https://codereview.chromium.org/2441213002
diff --git a/git_cl.py b/git_cl.py
index 5b0f00a..33517b9 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -4810,7 +4810,7 @@
 
   error_message = cl.CannotTriggerTryJobReason()
   if error_message:
-    parser.error('Can\'t trigger try jobs: %s')
+    parser.error('Can\'t trigger try jobs: %s' % error_message)
 
   if not options.name:
     options.name = cl.GetBranch()