Return an error when synchronize is used with something other than an ssh connection

Suggested in #2832

lib/ansible/plugins/action/synchronize.py

@@ -157,6 +157,13 @@ class ActionModule(ActionBase):
         except (AttributeError, KeyError):
             delegate_to = None
 
+        # ssh paramiko and local are fully supported transports.  Anything
+        # else only works with delegate_to
+        if delegate_to is None and self._play_context.connection not in ('ssh', 'paramiko', 'smart', 'local'):
+            result['failed'] = True
+            result['msg'] = "synchronize uses rsync to function. rsync needs to connect to the remote host via ssh or a direct filesystem copy. This remote host is being accessed via %s instead so it cannot work." % self._play_context.connection
+            return result
+
         use_ssh_args = self._task.args.pop('use_ssh_args', None)
 
         # Parameter name needed by the ansible module