service: log better warning when CallError raised.

mitogen/service.py

@@ -185,7 +185,7 @@ class Service(object):
             if response is not self.NO_REPLY:
                 msg.reply(response)
         except mitogen.core.CallError:
-            LOG.warning('%r: %s', self, msg)
+            LOG.warning('%r: call error: %s: %s', self, msg, e)
             e = sys.exc_info()[1]
             msg.reply(e)
         except Exception: