Handle errors from external_unlock & mysql_unlock_tables

Other things:
- Handler errors from ha_maria::implict_commit
- Disable DBUG in safe_mutex_lock to get trace file easier to read

mysql-test/main/myisam_debug.test

@@ -67,6 +67,8 @@ lock tables t1 write;
 insert t1 values (1),(2),(1);
 set @old_dbug=@@debug_dbug;
 SET debug_dbug='+d,mi_lock_database_failure';
+--error HA_ERR_CRASHED
 unlock tables;
+SHOW WARNINGS;
 SET debug_dbug=@old_dbug;
 drop table t1;