Emit error message if target of include is not found
If the target of an include directive is not found, Antora does not complain. The only indication that there is a problem is that the include directive is passed through to the output file unresolved.
Antora should log an error message to indicate that the target of the include was not found. The information in the message will be limited until #145 (closed) is resolved, but at least the information is captured (and available to a custom logger) and the user is given some indication that there's a problem.