core.tracklist: Remove pylint comment
core.tracklist: Test changed index() behavior
core.tracklist: Tweak docstrings
core.tracklist: Remove redundant if stmt in index()
core.tracklist: Rename mark_{consumed => played}
core.tracklist: Rename mark_{starting => playing}
core.tracklist: Add docstrings to mark_*
core.tracklist: Move logging into mark_unplayable()
core.tracklist: Improve if check in mark_unplayable()
core.tracklist: Formatting
tests: Formatting
jsonrpc: Test application error responses again

The test was modified to not fail after a refactoring, making it not test what
it was intended to test at all. This reverts the changes and updates the test
in another way, keeping the original intention.
docs: Update changelog for tidy-up-core changes