bot_update: improve UX when gclient is not configured.

recipe_modules/bot_update/api.py

Index: recipe_modules/bot_update/api.py
diff --git a/recipe_modules/bot_update/api.py b/recipe_modules/bot_update/api.py
index 42cf3ec094b18b7041daaef21d87a2750ab19caf..7650a342775115dfc247ca1103a90b5cc13cfbd6 100644
--- a/recipe_modules/bot_update/api.py
+++ b/recipe_modules/bot_update/api.py
@@ -78,6 +78,8 @@ class BotUpdateApi(recipe_api.RecipeApi):
     # We can re-use the gclient spec from the gclient module, since all the
     # data bot_update needs is already configured into the gclient spec.
     cfg = gclient_config or self.m.gclient.c
+    assert cfg is not None, (
+        'gclient_config or configured in advance gclient module is required')

[Bons, 2016/07/25 15:21:26]

this sentence is hard to follow. since i don't know 100% of what is required
it’s difficult for me to provide a better wording.

[tandrii(chromium), 2016/07/25 15:32:04]

How about now?

 
     # Used by bot_update to determine if we want to run or not.
     master = self._mastername