Commented Task: Throws an exception when no credentials are set [2229]

Currently Tweetinvi can fail at different time when the credentials are invalid. For the sake of developers, throw an exception informing them that they have not yet set credentials.
Comments: Associated with changeset 38141: Version 0.9.4.0
- Added TwitterCredentialsNullException that is thrown when requesting a Twitter API endpoint without specifying any credentials
- Update the Program project to use the latest naming convention for generating a TweetSearch parameter
- Renamed ExecuteMediaQuery into GenerateMultioartWebRequest to explain the goal of the method