command: forward database error on failure

If the database triggered a failure, it wasn't reported in the tests
because $this->error was not set. Conversely, the error was always
printed through dol_print_error() whose documentation explicitely states
that it should not be used for class and $this->error should be used
instead.

htdocs/commande/class/commande.class.php

@@ -1191,7 +1191,7 @@ class Commande extends CommonOrder
 
 			return 0;
 		} else {
-			dol_print_error($this->db);
+			$this->error = $this->db->lasterror();
 			$this->db->rollback();
 			return -1;
 		}