When config.trace=loads, log IOException for each load
on throwing ConfigException.IO include class name of the original exception

This is intended to surface "FileNotFoundException" or whatever.
Better error reporting in SimpleIncluder.fromBasename

 - if config.trace=loads, log if we don't find anything
 - keep the first load failure and use as 'cause' for the
   thrown exception

Together with the previous commits to log load errors, this
fixes #78