isValidAgainst doesn't override custom message

isValidAgainst method in assertj3.XmlAssert module doesn't override custom error message:

```
XmlAssert
.assertThat(xmlSource)
.withFailMessage("Alarm alarm!")
.isValidAgainst(xsdSource);
```

expected:

```
java.lang.AssertionError: Alarm alarm!
Expecting: ...
```

fact:

```
java.lang.AssertionError:
Expecting: ...
```

