Add an explanation in the default exception fetcher to avoid scream in the future...

Add an explanation in the default exception fetcher to avoid scream in the future as loud as I did yesterday; plus also display the current thread when displaying a backtrace. These messages often get desynchronized with other logs, making it harder to see which backtrace matches which log (which thread)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@3978 48e7efb5-ca39-0410-a469-dd3cf9ba447f