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.

Utility/Directory.hs

@@ -11,7 +11,6 @@ module Utility.Directory where
 
 import System.IO.Error
 import System.Directory
-import Control.Exception (throw, bracket)
 import Control.Monad
 import Control.Monad.IfElse
 import System.FilePath
@@ -114,7 +113,7 @@ moveFile src dest = tryIO (rename src dest) >>= onrename
 			whenM (isdir dest) rethrow
 			viaTmp mv dest undefined
 	  where
-		rethrow = throw e
+		rethrow = throwM e
 		mv tmp _ = do
 			ok <- boolSystem "mv" [Param "-f", Param src, Param tmp]
 			unless ok $ do