Clarify the failing change in MergeabilityChecker

If an update check fails, it may be change specific. Record the
change as part of the logged message.

Change-Id: I33aba050572f919e150d8e0f1bd678f575301b72

gerrit-server/src/main/java/com/google/gerrit/server/change/MergeabilityChecker.java

@@ -350,10 +350,10 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
         // change is closed
         return false;
       } catch (Exception e) {
-        String msg = "Failed to update mergeability flags for project "
-            + change.getDest().getParentKey() + " on update of "
-            + change.getDest().get();
-        log.error(msg, e);
+        log.error(String.format(
+            "cannot update mergeability flag of change %d in project %s after update of %s",
+            change.getId().get(),
+            change.getDest().getParentKey(), change.getDest().get()), e);
         throw e;
       } finally {
         tl.setContext(old);