local: Temporary workaround for issue #527

Adds a fallback to `Track(uri=uri` when track lookup fails for playlists. This
means we can at least load metadata less tracks giving users functioning
playlists, instead of only supporting `local:track:...` style URIs.

Issue is not fixed, but this is sufficient to reduce priority until we get to
the larger planed refactor for this and other core API issues.
local: Add test for fallback handling of unknown track uris in playlists

- Checks that adding a non local:track:... uri to a playlist and reading it
  back works.
- Also fixes mistake in API usage where we assumed lookup still raised
  LookupError.
Merge pull request #539 from adamcik/fix/bug-527-temp-workaround

local: Temporary workaround for issue #527