MilestoneOperation hint is incorrect if milestone doesn't exist

[Ryan J Ollos]

The following hint could be omitted if the milestone doesn't exist:

Also, some minor changes would likely make the plugin more robust. If the milestone attribute is omitted while specifying the set_milestone operation for the action, a traceback results:

File "/home/user/Workspace/t11944/teo-rjollos.git/trac/web/main.py", line 551, in _dispatch_request
  dispatcher.dispatch(req)
File "/home/user/Workspace/t11944/teo-rjollos.git/trac/web/main.py", line 225, in dispatch
  resp = chosen_handler.process_request(req)
File "/home/user/Workspace/t11944/teo-rjollos.git/trac/ticket/web_ui.py", line 175, in process_request
  return self._process_ticket_request(req)
File "/home/user/Workspace/t11944/teo-rjollos.git/trac/ticket/web_ui.py", line 687, in _process_ticket_request
  get_reporter_id(req, 'author'), field_changes)
File "/home/user/Workspace/t11944/teo-rjollos.git/trac/ticket/web_ui.py", line 1722, in _insert_ticket_data
  self._get_action_controls(req, ticket)
File "/home/user/Workspace/t11944/teo-rjollos.git/trac/ticket/web_ui.py", line 450, in _get_action_controls
  req, ticket, action)
File "/home/user/Workspace/t11944/tracdev/plugins/MilestoneOperation.py", line 71, in render_ticket_action_control
  res_ms = self.__get_resolution_milestone_dict(ticket, action)
File "/home/user/Workspace/t11944/tracdev/plugins/MilestoneOperation.py", line 125, in __get_resolution_milestone_dict
  res_milestone[res] = transition[1]