Add some helpful detection if core modules are not found for those running on…

Add some helpful detection if core modules are not found for those running on the development branch.

lib/ansible/runner/__init__.py

@@ -1215,7 +1215,11 @@ class Runner(object):
         module_suffixes = getattr(conn, 'default_suffixes', None)
         module_path = utils.plugins.module_finder.find_plugin(module_name, module_suffixes)
         if module_path is None:
+            module_path2 = utils.plugins.module_finder.find_plugin('ping', module_suffixes)
+            if module_path2 is not None:
                 raise errors.AnsibleFileNotFound("module %s not found in configured module paths" % (module_name))
+            else:
+                raise errors.AnsibleFileNotFound("module %s not found in configured module paths.  Additionally, core modules are missing. If this is a checkout, run 'git submodule update --init --recursive' to correct this problem." % (module_name))
 
 
         # insert shared code and arguments into the module