http: Add missing string interpolation placeholder

docs/changelog.rst

@@ -13,6 +13,10 @@ v0.19.1 (UNRELEASED)
   Mopidy continue to work on Debian/Raspbian stable, where Tornado 2.3 is the
   newest version available.
 
+**HTTP**
+
+- Add missing string interpolation placeholder.
+
 **Development**
 
 - ``mopidy --version`` and :meth:`mopidy.core.Core.get_version` now returns the

mopidy/http/handlers.py

@@ -144,7 +144,7 @@ class JsonRpcHandler(tornado.web.RequestHandler):
                     'Sent RPC message to %s: %r',
                     self.request.remote_ip, response)
         except Exception as e:
-            logger.error('HTTP JSON-RPC request error:', e)
+            logger.error('HTTP JSON-RPC request error: %s', e)
             self.write_error(500)
 
     def set_extra_headers(self):