Add diagnostic output to HtmlDecorator if AJ-Doc generation fails

Add diagnostic output to HtmlDecorator if AJ-Doc generation fails

HtmlDecorator.decorateHTMLFile is where after Java version upgrades
(i.e. also new Javadoc generator version) usually tests fail for the
first time during builds because strings no longer match as expected.

There now is this log message on stdOut: "Something unexpected went
wrong in HtmlDecorator. Here is the full file causing the problem:"
After that, a full HTML page is logged. I hope this helps me identify
the new error on GitHub Linux Java 16, because the same test works on
Windows and I have no idea how to remote-debug a GitHub CI build.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>