Clarify error message

src/libstat/stat_process.c

@@ -807,8 +807,8 @@ rspamd_stat_learn (struct rspamd_task *task,
 	}
 
 	if (!learned) {
-		g_set_error (err, rspamd_stat_quark (), 500, "message cannot be learned"
-				" for any classifier defined");
+		g_set_error (err, rspamd_stat_quark (), 500, "message cannot be learned as "
+				"it has too few tokens for any classifier defined");
 	}
 	else {
 		g_atomic_int_inc (&task->worker->srv->stat->messages_learned);