Updated RedditException to inherit from System.Exception and provide more standardized constructors

The previous version of this file didn't inherit from System.Exception which meant you couldn't even throw it. This format is the same as the Exception snippet in VS and also allows you to serialize it if you need to (which probably isn't necessary but is generally standard for exceptions.)
Merge pull request #33 from Elite6809/patch-1

Updated RedditException to inherit from System.Exception and provide more standardized constructors