Refactor errors.

- Add some tests to assure error raising from client is correct
- Define the current errors under the YouTubeIt namespace
Introduce ResourceNotFoundError.

- Is raised whenever the api responds with 404
- Used to raise UploadError which made no sense at all for a missing video etc
Add changelog with info about changes to error handling.
Merge pull request #182 from dbackeus/exceptions

Better Exceptions