PlayBook: better error message

When a PlayBook has missing arguments the error message now shows which ones are missing.

lib/ansible/playbook/__init__.py

@@ -91,8 +91,17 @@ class PlayBook(object):
 
         self.SETUP_CACHE = SETUP_CACHE
 
-        if playbook is None or callbacks is None or runner_callbacks is None or stats is None:
-            raise Exception('missing required arguments')
+        arguments = []
+        if playbook is None:
+            arguments.append('playbook')
+        if callbacks is None:
+            arguments.append('callbacks')
+        if runner_callbacks is None:
+            arguments.append('runner_callbacks')
+        if stats is None:
+            arguments.append('stats')
+        if arguments:
+            raise Exception('PlayBook missing required arguments: %s' % ', '.join(arguments))
 
         if extra_vars is None:
             extra_vars = {}