local: Add friendly help if no local library cache found

Fixes #711

docs/changelog.rst

@@ -121,6 +121,10 @@ Feature release.
 
 **Local backend**
 
+- The JSON local library backend now logs a friendly message telling you about
+  ``mopidy local scan`` if you don't have a local library cache. (Fixes:
+  :issue:`711`)
+
 - The ``local scan`` command now use multiple threads to walk the file system
   and check files' modification time. This speeds up scanning, escpecially
   when scanning remote file systems over e.g. NFS.

mopidy/local/json.py

@@ -20,6 +20,14 @@ logger = logging.getLogger(__name__)
 
 # TODO: move to load and dump in models?
 def load_library(json_file):
+    if not os.path.isfile(json_file):
+        logger.info(
+            'No local library metadata cache found at %s. 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.',
+            json_file)
+        return {}
     try:
         with gzip.open(json_file, 'rb') as fp:
             return json.load(fp, object_hook=models.model_json_decoder)