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.

Command/Move.hs

@@ -152,17 +152,17 @@ fromOk src key = go =<< Annex.getState Annex.force
 fromPerform :: Remote -> Bool -> Key -> AssociatedFile -> CommandPerform
 fromPerform src move key afile = moveLock move key $
 	ifM (inAnnex key)
-		( handle move True
-		, handle move =<< go
+		( dispatch move True
+		, dispatch move =<< go
 		)
   where
 	go = notifyTransfer Download afile $ 
 		download (Remote.uuid src) key afile noRetry $ \p -> do
 			showAction $ "from " ++ Remote.name src
 			getViaTmp key $ \t -> Remote.retrieveKeyFile src key afile t p
-	handle _ False = stop -- failed
-	handle False True = next $ return True -- copy complete
-	handle True True = do -- finish moving
+	dispatch _ False = stop -- failed
+	dispatch False True = next $ return True -- copy complete
+	dispatch True True = do -- finish moving
 		ok <- Remote.removeKey src key
 		next $ Command.Drop.cleanupRemote key src ok