Adding a check for a valid JSON response from the weather API in !weather

In the event that the openweathermap.org API doesn't return a valid JSON response, the weather script will throw a ValueError exception.

![Error Image](http://content.screencast.com/users/mikedavis.webfilings/folders/Jing/media/0997ed5e-f57b-4728-a82d-2ea95ec46913/00000047.png)

This adds a check to the json load and exits in the event that the load call throws a ValueError.

![Fixed Image](http://content.screencast.com/users/mikedavis.webfilings/folders/Jing/media/20fb453f-5f87-40a7-8cc8-4ffc0dbdd9bb/00000049.png)