java_model: more informative error msg for enum lookup failures

They are still triggered for a ton of different cases, where
the lookup target isn't really an enum in the first place.

This should be fixed.
diff --git a/java_gen/java_model.py b/java_gen/java_model.py
index 8726632..fa2199c 100644
--- a/java_gen/java_model.py
+++ b/java_gen/java_model.py
@@ -815,7 +815,7 @@
             entry = enum.entry_by_version_value(self.msg.version, self.value)
             return "%s.%s" % ( enum.name, entry.name)
         except KeyError, e:
-            logger.debug("No enum found", e)
+            logger.debug("No enum found for type %s version %s value %s", java_type, self.msg.version, self.value)
             return self.value
 
     @property