Change mtr_error to mtr_warning allowing processing to continue when mysqld fails to start

mysql-test/mysql-test-run.pl:
  Report warining when failure to start mysqld was detected, this will allow the testcase to continue executing.
  The failure to start mysqld will be caugt when executing mysqltest

mysql-test/mysql-test-run.pl

@@ -3355,7 +3355,7 @@ sub run_testcase_start_servers($) {
 
     if (mysqld_wait_started($mysqld))
     {
-      mtr_error("Failed to start $mysqld->{'type'} mysqld $mysqld->{'idx'}");
+      mtr_warning("Failed to start $mysqld->{'type'} mysqld $mysqld->{'idx'}");
     }
   }
 }