Content service: log error when throwing InternalServerErrorException

The Experience Manager uses Content service module.

Internally, for instance in org.onehippo.cms.channelmanager.content.document.DocumentsServiceImpl, sometimes  InternalServerErrorException is thrown:

throw new InternalServerErrorException(new ErrorInfo(Reason.SERVER_ERROR));

This usually goes accompanied by DEBUG, INFO, WARN log statements, but let's make sure an ERROR level log is always done here. 

Reason is that in case ERROR level is the default, it is hard to find out what the reason is of the error. You just get an "500 Server Error".