Noticed in Issue2133; when a patch fails to apply, the Prelude.catch in
applyHashed (src/Darcs/Repository/HashedRepo.hs) catches the exception,
leading to an unnecessary (and misleading) "Doing a one-time conversion"
We should ensure that we only perform a conversion if we know we need to
(probably some specific exceptions needed, but I've not really thought
about where other exceptions are coming from, and in what instances we
want to convert).