Provide named error for calling SkygearContainer that is init before the transport ready

[rickmak]

Developer will write code like this.

container = SkygearContainer()

@skygear.op('booking:new', user_required=True)
def new_booking(info):
    container.send_action('record:save', {})

Since v0.24.0, default SkygearContainer.send_action is injected after py-skygear init and load the config from envvar. If the user created an instance before the config finished, the transport can be None and calling of send_cation will result in following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.4/site-packages/skygear/transmitter/common.py", line 40, in wrapper
    return dict(result=f(self, *args, **kwargs))
  File "/usr/local/lib/python3.4/site-packages/skygear/transmitter/common.py", line 102, in call_func
    return self.hook(obj, param)
  File "/usr/local/lib/python3.4/site-packages/skygear/transmitter/common.py", line 180, in hook
    returned = func(record, original_record, conn)
  File "/usr/local/lib/python3.4/site-packages/skygear/decorators.py", line 85, in hook_func
    func(record, original_record, db)
  File "./skygear_plugins/tailor_image_resizing.py", line 104, in after_save_product_image
    DB_TABLES.get('product')
  File "./skygear_plugins/tailor_image_resizing.py", line 136, in resize_image_and_save_to_skygear_db
    image_url = get_image_url(record, field, table)
  File "./skygear_plugins/tailor_image_resizing.py", line 197, in get_image_url
    response = container.send_action('record:query', data)
  File "/usr/local/lib/python3.4/site-packages/skygear/container.py", line 106, in send_action
    resp = self.transport.send_action(action_name, payload, url, timeout)
AttributeError: 'NoneType' object has no attribute 'send_action'

• Skygear Server Date/Version: v1.1.0
• Application Platform:
• Is this a regression?
• Attached logs, screenshots

Expected Results

Tell the dev the transport is not ready at the instantiation time.

Actual Results

Meaningless error message of AttributeError.