conf: domainlist: Explicitly report failure to load domain config

When dropping a domain report which one was dropped so that it's not
necessary to rummage through the logs.

src/conf/virdomainobjlist.c

@@ -605,6 +605,8 @@ virDomainObjListLoadAllConfigs(virDomainObjListPtr doms,
             if (!liveStatus)
                 dom->persistent = 1;
             virObjectUnlock(dom);
+        } else {
+            VIR_ERROR(_("Failed to load config for domain '%s'"), entry->d_name);
         }
     }