Add an error message when explicitly specifying bots without specifying the master

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index eaed4baf2291a4d8822987485ef91425df000e73..82d5a78abb6f5780fe0dafcca334cf03353e8a1a 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2184,6 +2184,10 @@ def CMDtry(parser, args):
   if not options.name:
     options.name = cl.GetBranch()
 
+  if options.bot and not options.master:
+    parser.error('For manually specified bots please also specify the '
+                 'tryserver master, e.g. "-m tryserver.chromium".')
+
   def GetMasterMap():
     # Process --bot and --testfilter.
     if not options.bot: