Add a nicer log entry when an invalid lookup URI is requested (#17)

Erinrequested to merge
covertneko/mopidy:issue-17-invalid-lookup-uris into master

This fixes #17 (closed), which is a frequent issue if Iris is being used as a frontend. This is also related to one of the errors mentioned in #16 (closed).

I also fixed a duplicated test name that was resulting in another test not being run.

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