Improve error reporting for WCAG tests

It is very hard to find and understand where the WCAG problem is in the page. There is no screenshot, no line number in the code source.

See currentWCAG.png for error messages.

An improvement would be to provide at least the line number, similar to what we do for DefaultValidationTest.java, see https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-distribution/xwiki-platform-distribution-flavor/xwiki-platform-distribution-flavor-test/xwiki-platform-distribution-flavor-test-webstandards/src/test/it/org/xwiki/test/webstandards/framework/DefaultValidationTest.java#L132-L158