`YAMLMapper` doesn't close output file if an error occurs

If an exception is thrown while writing anything to a YAML file, the mapper won't close the opened YAML file.

A simple example to reproduce the resource leak:

```Java
@Test
public void test() throws IOException {
	try {
		new YAMLMapper().writeValue(new File("test.yml"), new Object());
	} finally {
		Files.delete(Paths.get("test.yml"));
	}
}
```