Treat all fatal XR schematron rules as errors, not as warnings.

In org.mustangproject.validator.XMLValidator.validateSchematron(),
the severity has to be determined for every node of the validation
result.  Otherwise, all schematron assertions tagged as "fatal"
will be treated as warnings after the first warning.

validator/src/test/java/org/mustangproject/validator/ZUGFeRDValidatorTest.java

@@ -216,10 +216,10 @@ public class ZUGFeRDValidatorTest extends ResourceCase {
 
 		assertThat(res).valueByXPath("count(//error)")
 			.asInt()
-			.isEqualTo(1);
+			.isEqualTo(2);
 		assertThat(res).valueByXPath("count(//warning)")
 			.asInt()
-			.isEqualTo(3);
+			.isEqualTo(2);
 
 		assertThat(res).valueByXPath("count(//notice)")
 			.asInt()