plcont: Subclass collections.Sequence
Revert "plcont: Make repr() look like a list"

This reverts commit e72b407546cdbd76cca53b71f2edf2a2dc5842b6.
plcont: Show typo *and* show collection content
plcont: Reject any combination of whitespace-only playlist names
plcont: Add TODO on finding a better exception type
plcont: Raise TypeError instead of ValueError