unify exception handling into Utility.Exception

Removed old extensible-exceptions, only needed for very old ghc.

Made webdav use Utility.Exception, to work after some changes in DAV's
exception handling.

Removed Annex.Exception. Mostly this was trivial, but note that
tryAnnex is replaced with tryNonAsync and catchAnnex replaced with
catchNonAsync. In theory that could be a behavior change, since the former
caught all exceptions, and the latter don't catch async exceptions.

However, in practice, nothing in the Annex monad uses async exceptions.
Grepping for throwTo and killThread only find stuff in the assistant,
which does not seem related.

Command.Add.undo is changed to accept a SomeException, and things
that use it for rollback now catch non-async exceptions, rather than
only IOExceptions.

Annex/Environment.hs

@@ -13,7 +13,6 @@ import Common.Annex
 import Utility.UserInfo
 import qualified Git.Config
 import Config
-import Annex.Exception
 
 #ifndef mingw32_HOST_OS
 import Utility.Env
@@ -58,7 +57,7 @@ checkEnvironmentIO =
 {- Runs an action that commits to the repository, and if it fails, 
  - sets user.email and user.name to a dummy value and tries the action again. -}
 ensureCommit :: Annex a -> Annex a
-ensureCommit a = either retry return =<< tryAnnex a 
+ensureCommit a = either retry return =<< tryNonAsync a 
   where
   	retry _ = do
 		name <- liftIO myUserName