Fixed GGTwitter:post callback event error

When posting a tweet I was always getting a "failed" event in the callback after a successful post.

The oAuth.rawPostRequest() function returns an isError variable as the first return value which caused confusion in the callback logic in GGTwitter.
 
I changed the ambiguous variable name 'status' to 'isError' to reflect its use in all callbacks.
I also changed the GGTwitter:post callback logic to handle the event properly.