main: Validate extension's distribution requirements

mopidy/__main__.py

@@ -144,7 +144,15 @@ def load_extensions():
     extensions = []
     for entry_point in pkg_resources.iter_entry_points('mopidy.extension'):
         logger.debug('Loading extension %s', entry_point.name)
+
+        try:
             extension_class = entry_point.load()
+        except pkg_resources.DistributionNotFound as ex:
+            logger.info(
+                'Disabled extension %s: Dependency %s not found',
+                entry_point.name, ex)
+            continue
+
         extension = extension_class()
 
         # TODO Validate configuration, filter out disabled extensions

setup.py

@@ -45,12 +45,12 @@ setup(
             'mopidy-scan = mopidy.scanner:main',
         ],
         b'mopidy.extension': [
-            'http = mopidy.frontends.http:Extension',
-            'lastfm = mopidy.frontends.lastfm:Extension',
+            'http = mopidy.frontends.http:Extension [http]',
+            'lastfm = mopidy.frontends.lastfm:Extension [lastfm]',
             'local = mopidy.backends.local:Extension',
             'mpd = mopidy.frontends.mpd:Extension',
             'mpris = mopidy.frontends.mpris:Extension',
-            'spotify = mopidy.backends.spotify:Extension',
+            'spotify = mopidy.backends.spotify:Extension [spotify]',
             'stream = mopidy.backends.stream:Extension',
         ],
     },