Catch and log layer-specific exceptions when GWC searches for layers that have been affected by a style change.
Otherwise the cached styles will not be cleared out if even a single layer has a configuration problem that throws an exception.
Merge pull request #337 from cga-harvard/gwc_update_catch

Catch and log layer-specific exceptions when GWC searches for layers associated with a style
Merge branch 'master' of git://github.com/geoserver/geoserver