Do not log ERROR for XhtmlException when getting macro body, log DEBUG

In the MultiExcerptHelper we have a logline that logs ERROR for some situations where Confluence executes the macro for a state in which the body of some macros on the page do not parse, maybe during page editing.

The multiexcerpt macro works in this situation, but there is an exception that is thrown by Confluence invoking our macro and we catch that exception.

This logline was added in a recent release of Multiexcerpt.

The MultiExcerptHelper should log DEBUG for that case. This will keep the log clean and will only show the logline when a user intentionally enables the DEBUG log level. The logline is only meant as a safety feature to help us in the future if Confluence changes its behavior in this area.