local: Add friendly help if no local library cache found

Fixes #711

Example output before:

```
INFO     Loaded 0 local playlists from /home/jodal/.local/share/mopidy/local/playlists
WARNING  Loading JSON local library failed: [Errno 2] No such file or directory:
'/home/jodal/.local/share/mopidy/local/library.json.gz'
INFO     Loaded 0 local tracks using json
```

Example output now:

```
INFO     Loaded 0 local playlists from /home/jodal/.local/share/mopidy/local/playlists
INFO     No local library metadata cache found at
/home/jodal/.local/share/mopidy/local/library.json.gz. Please run `mopidy local scan` to
index your local music library. If you do not have a local music collection, you can disable
the local backend to hide this message.
INFO     Loaded 0 local tracks using json
```