Stricter response validation

[simon-weber]

At the moment, only the structure and type of the responses are validated in protocol.WC_Protocol. However, Validictory has a whole host of other tools to use (some undocumented), and the stricter validation is, the sooner a protocol change would become apparent.

Some simple things to validate:

• for playlists, continuation should always be False - this tests the playlist limit assumption
• any kind of id (song, playlist, entry) can be matched against a regex, like is currently done in test.utils

Also, it's probably smart to prompt end-users to submit an issue when validations fail.