Fixes https://github.com/yesodweb/yesod/issues/1771
This is done by adding a function to Yesod
typeclass which can match on any exception
and tell the framework if it should rethrow
or not.
I used an overridable function because it seemed
more flexible then a whitelist.
A user can now for example choose to throw
everything, or catch everything as easily.
add docs
bump
