Added assert_not_raises

Added this for certain cases of error handling in my code where it should capture and handle a specific exception being thrown. 

Example:
I have a request object that makes a request, but the http request fails and throws an exception because the remote host is not available, my code should handle this specifically and provide default values for when this specific case happens.

To test this requires that I have it make a failing request to a non-existent remote host and I make sure that the error is not thrown.