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/View.hs

@@ -410,19 +410,19 @@ withViewChanges addmeta removemeta = do
   where
 	handleremovals item
 		| DiffTree.srcsha item /= nullSha =
-			handle item removemeta
+			handlechange item removemeta
 				=<< catKey (DiffTree.srcsha item) (DiffTree.srcmode item)
 		| otherwise = noop
 	handleadds makeabs item
 		| DiffTree.dstsha item /= nullSha = 
-			handle item addmeta
+			handlechange item addmeta
 				=<< ifM isDirect
 					( catKey (DiffTree.dstsha item) (DiffTree.dstmode item)
 					-- optimisation
 					, isAnnexLink $ makeabs $ DiffTree.file item
 					)
 		| otherwise = noop
-	handle item a = maybe noop
+	handlechange item a = maybe noop
 		(void . commandAction . a (getTopFilePath $ DiffTree.file item))
 
 {- Generates a branch for a view. This is done using a different index