Make error more prominent

[guenhter]

Please provide details about:

• What you're trying to do
  In big documents, it is often hard to see that a include link is broken. The preview shows

Unresolved directive in <stdin> - include::foo.adoc[]

but this is very easy to miss when scrolling through the document.

• How you think this extension could handle this

It would be great if errors are somehow more prominent and easier to see